Abstract: To analyze the causes of the drought over northwea China,first of all,the wet and dry year criterion and their division on the north side of Tibetan Plateau(NTP) centered the northwest China drought area have been made utilizing the precipitation data of five stations over the area during the period from June to August of 1952-1990 in this paper. Then,the differences between the vertical circulation over NTP in wet and dry summer have been comparatively analyzed using the gridded data of ECMWF from 1979 to 1986. At last,the connection between the surface thermal characteristic of the Plateau and the wet and dry summer over NTP was discussed as well.The main results show that:1)the wet and dry summer criterion with precipitation standard deviation is proper for the drought area of northwest China;2)there are clear differences between the vertical circulations over NTP and the Plateau in wet and dry summer:in wet summer upward motion over NTP dominates and the upward motion over the Plateau is stronger,but vice versa in dry summer;3)the wet and dry summer of NTP is closely related to the surface thermal condition of the Plateau,namely,the heating effect of the Plateau surface is stronger in wet summer than in dry summer.