To better understand the features of distribution and variation of the atmospheric mixed layer depth( hereafter ML) and the impact of the deep ML on the dry climate over Northwest China( NWC), in this paper, the ML thickness, its change, and the impact of it on the NWC dry climate formation have been calculated and analyzed, utilizing the observed sounding data in Zhangye and Minqin in summerand related months in more years. The main conclusions are as follows: (1) There are clear annual changes and inter-local differences for the ML thickness in Zhangye and Minqin. On the average, the ML depth is the shallowest with just less than 1 km in winter； the deepest with 3~4 km, in summer half year( especially in May and June before the local relative rainy period ). The ML depth is much higher than that in humid- and semi-humid areas in East China, and also is equivalent to or even higher than that in Qinghai-Xizang Plateau in summer. Further, the ML depth are higher in far north and west in Hexi Corridor of Gansu, where are near to the center of dry area of NWC. (2) The dry ( wet) weather has largely influence on the ML depth in arid area over NWC summer through intensifying(weakening) the exchange of sensible heat and dry convection . The statistic shows that the deepest averaged ML depth can be as high as 3413 m on the high temperature days； the next with 2838 m, on the dry days； the third with 2261 m, on the light rain ones； the shallowest with just 1942 m, on the medium-and strong-rain days. Consequently, in comparison to wet summers, the ML depth in dry summers is about 300 m highers. (3) Not only the dry and wet weather in dry area summer can considerably influence the ML depth, but also through the evaporation depletion of rain drop, the deep ML over the arid land in summer half year reduce, in turn, precipitation and intensify its dry climate. Thus, the deep ML over arid area in summer half year is one of factors of NWC drought climate formation. Some existed evidences show the conclusions above have the representative for entire NWC(even China-Mongolia) dry land for whole summer half year(May~September, not just June-August). Certainly some further investigation is still needed as well.