Based on the observation and reanalysis data during 1961–2014, the spatial and temporal features of the dry and wet changes in summer and autumn in Northwest China (35°-50°N, 75°-95°E) are investigated. We examine the contribution of evapotranspiration and precipitation to the linear trend of dryness and wetness. Meanwhile, the large-scale atmospheric circulations and water vapor budget related to interdecadal characteristics of dryness and wetness are analyzed. Results show that the drought variability in summer and autumn in Northwest China is the largest in the four seasons, and these two seasons are the highest probability of drought. However, there was a significant increasing trend of wetness in Northwest China from 1961 to 2014, in which evapotranspiration and precipitation play a significant role in the upwards trend in humidity over Northwest China. The increase in precipitation and the decrease in evapotranspiration both positively contribute to the trend of wetness in Northwest China. The total contribution rate of the two trends is 93.4% in summer and 67.5% in autumn. The interdecadal shift from dry to wet happened in summer and autumn around 1987 over Northwest China. Since 1987, the interdecadal humidification was mainly affected by changes in evapotranspiration and precipitation over Northwest China in summer. The decrease in evapotranspiration has a positive contribution to the humidification of the area, which is caused by the decrease in surface wind speed. The anomalous convergence of water vapor transport flux has led to an increase in precipitation in Northwest China. The water vapor diagnostic analysis further shows that the increase in precipitation mainly comes from the enhancement of local evaporation, with a contribution rate of 80%， indicating that local evaporation is an essential source of water vapor for precipitation. In addition, the summer water vapor advection term is positive (that is, water vapor flux convergence is strengthened), which has a positive contribution to the increase in precipitation. This contribution is mainly caused by the dynamic component related to wind speed. In comparison, the decrease in net radiation flux and surface wind speed simultaneously led to a decrease in evapotranspiration in Northwest China in autumn after 1987, which caused interdecadal humidification in the region.