By using data of Precipitation Radar (PR) and Microwave Imager (TMI) sensors on the Tropical Rainfall Measuring Missions (TRMM), and intensive observational sounding data, the evolution characteristics and long-lasting development mechanism of mesoscale convective clouds over the northern South China Sea (15°N-25°N, 108°E-122°E) in the South China Sea Monsoon Experiment (SCSMEX) are studied. The results show that the cold frontal cloud band from southern China coastal region started to weaken and dissipate as it entered the South China Sea before onset of the monsoon, and intensified and developed into deeper and long-lasting convective clouds with heavy precipitation after onset of the monsoon. The convective available potential energy (CAPE), wind shear and latent heat were found to play very important roles in maintaining and developing of the intense convective clouds. Before the monsoon onset, though the atmosphere over the northern South China Sea had higher CAPE, the wind shear was too weak, thus, the convective cloud band could not last longer. During the monsoon period, the monsoon circulation maintained the high CAPE and wind shear, and led to a long-lasting development of intense convective cloud system. The heating rate due to latent heat release also increased more than two times during the monsoon period, which provided the favorable condition for upward transportation of energy and water vapor from the ocean.