Abstract: The data from core, slice, SEM and XRD were used to study the characteristics of the Jurassic reservoirs in Santanghu Basin. In the Lower-Middle Jurassic low-pressure reservoir, lots of feldspars were dissolved, the content of kaolinite is from 3% to 16%, and the average value is 9.6%. During the formation of kaolinite, the formation water was abundantly consumed, and fluid content in pores decreased. The porous space increased by 1%, and the average value of fluid pressure almost decreases by 2MPa during feldspar dissolution. So, feldspar dissolution is the most important factor for declining of the reservoir pressure in the Lower-Middle Jurassic reservoir of Santanghu Basin.