The land-use and land-cover change has a significant impact on the climate at different spatio-temporal scales. In this study, we explored the long term oasis expansion effects on regional summer precipitation in the north slope of Tianshan Mountains, China using high-resolution regional climate model. The results indicate that the oasis expansion increases the summer precipitation in the middle Tianshan Mountains while it has only a small effect over the oasis regions itself. The results indicate further that the oasis expansion affects mainly the late afternoon summer convective precipitation. The advection of air with additional moisture from the oasis areas to the mountains due to the mountain/plain circulation system during the day triggers the orographic precipitation in the middle mountain regions. These new results indicate that the oasis expansion could attribute significantly to the recent finding from observational studies about the increasing trend of precipitation in the middle Tianshan Mountains.