In this paper, wind profile radar, runway automatic observation and Doppler weather radar were used to analyze the occurrence and evolution characteristics of a dry downburst (referred to as "5.14" in this paper) under a weak synoptic scale forcing in the semi-arid region of Lanzhou on May 14, 2020. The mesoscale numerical model WRF (Weather Research and Forecasting) is used to simulate the formation, movement and evolution characteristics of the hydrocondensate over the divergence outflow area of the process, and the possible mechanism of the outflow propagation of the process 5.14 is discussed. The results show that the lifetime of "5.14" process is about 30 min, and the cloud top height is more than 9 km. At the height of 3-6 km to the rear of the cloud body, the sudden intrusion of dry cold air jet, the fracture of cloud body, the collapse of cloud top, the downward transmission of momentum and the divergence of outflow jet at the height of 1-4 km in the middle and low altitude are the possible causes of the outflow of downburst. Snow crystals collide with supercooled cloud droplets to freeze and merge, resulting in a rapid growth of graupel particles in both sinking and outflow areas. The simulated mixing of graupel particles is 105 times larger than that during the outbreak of downburst. Graupel particles in sinking zone accelerate the formation of cold pools in clouds, which is one of the reasons for the excitation of strong downdraft. With the movement of the cloud body, the strong downdraft generates divergence outflow on the ground, and the intersection of the adjacent divergence outflow leads to the convergence upward movement of the airflow. Two updraft areas are formed on both sides of the downdraft at the front of the cloud body movement direction. With the deepening of the cold and dry inflow jet, two vertical circulations are generated in the direction of cloud body movement. The vertical circulation is formed by an updraft and a wet downdraft in the cloud. The wet downdraft in the vertical circulation forms cold pool diffusion near the surface, which promotes the burst of downburst and stimulates the gust front. The gust front diffused in the outflow direction of the downward-hitting storm divergence center, and the warm and wet updrafts in front of the gust front helped the new units merge into the original storm, and the storm was strengthened. With the advance of the gust front, the warm and wet updrafts were cut off, leading to the sinking of the heavy cold cloud top, and the continuous upward-rushing and collapsing of the cloud top formed the outflow propagation process of the downward-hitting storm. Rainwater particles transported by updraft in front of gust front are frozen near the 0 ℃ temperature layer. Heat released in the freezing process leads to the continuous rise of 0 ℃ temperature layer in the process of outflow propagation, and the melting layer of graupel particles sinking in the clouds rises. The evaporation layer of rainwater particles formed after melting increases in the falling process. As graupel particles melt and rainwater evaporates and absorbs heat in the process of falling, cold pools near the ground are constantly strengthened, leading to increased surface wind speed in the process of divergence propagation, which is an important reason for the formation of surface gales in the outflow propagation of downburst. The updraft, on the other hand, heats the atmosphere by condensation and enhances the upward motion. The development of downdraft contributes to the formation and maintenance of convective circulation and cold pool. After the formation of the downburst, the vertical closed circulation formed continuously in the direction of cloud body movement is the moving mechanism of the divergence center of the downburst. Due to the convergence and uplift effect of the outflow of the surface cold pool, the range of the updraft zone in the moving direction keeps increasing, and the vertical closed circulation cannot be regenerated due to the blockage of the updraft zone. At the same time, the eastward movement of the cloud body maintains the weakening of the circulation of water condensate in the vertical closed circulation structure of the downburst, which dissipates the vertical closed circulation structure and leads to the weakening and extinction of the divergence center of the downburst. Compared with previous studies, this dry downburst also has such features as inflow jet at the back of cloud body, radar echo reflectivity factor nuclear decline, momentum downflow, high water content of graupel particles, heat absorption and cold pool formation in the melting and evaporation process of water condensate. However, there is an obvious vertical closed circulation in the divergence center of dry downburst, which is the starting and maintaining mechanism of the divergence center of downburst. At the same time, the divergence center of downburst is closely related to the formation of gust front, and the gust front process is the main reason for the outflow propagation of dry downburst to form surface gales.