A non-hydrostatie meso-and micro-scale numerical model including detailed physica process both on ground surface an d in water is developed to sim ulate the distribution of the air specific humidity in the vicinity inlan d water body. The results are in agreement w ith observed data show ing that in the vicinity of water body , air specific hum idity increasea both in summer and in whiter, and the increasing ran ge is greater in winter than in sum mer.in sunny day than in cloudy day, and in nightthne than in daytime.When airflow passes through water body, the chan ge of air specific hum idity mainly occurs within original 2- 3km. Accordin g to power rule on both shores, the air specific hum idity approaches to that over land, w ithin 4 km on the win dward shore and beyond 20 km oii the leeward shore, the top. altitude where water body m ay affect air specific humidity is 200-doom .