This study examines the inter-annual variation of the difference in summer precipitation over the east and west side of the Ailao Mountain， as well as its relationship between the East Asia summer monsoon and the South Asia summer monsoon based on in situ observations， reanalysis products and model sensitivity experiments.As the result， climatological statistics show that there is more precipitation on the west side than on the east side of the Ailao Mountain.Compared to the total summer precipitation amount observed over five pairs of state-level weather stations， the difference is higher on the north part than on the south part， with negative correlations to some extent.The observed differences in precipitation are generally with a negative correlation between the East Asia summer monsoon index， and with a positive correlation between the South Asia summer monsoon index.The linear correlation coefficients with the index of monsoon interface， which stands for the relative strengths of the East Asia and South Asia monsoons， are higher than those with one single monsoon.With one positive and one negative year selected based on the index of monsoon interface， we design and conduct a set of model sensitivity experiments through changing the boundary condition of wind and water vapor fields respectively to quantify the impact on precipitation difference of monsoons.The result shows that the anomaly in the wind field is the major contributing factor of the difference in precipitation on the two sides of the Ailao Mountain.On the other hand， the anomaly in the water vapor field makes synchronized changes in precipitation on both sides of the mountain， while it contributes little to the precipitation difference.