Abstract: The main diagenesis of the middle and upper Jurassic reservoirs in Turpan Depression is studied by means of the casting thin sections, scanning electron microscope, X-ray diffraction and physical properties. Compaction and cementation are the major factors turning the pore texture of reservoir worse; while dissolution is the major factor making the pore texture of reservoir better. Dissolution transformed the physical properties of reservoirs. Modified by dissolution, the clastic rock of Sanjianfang-Xishanyao Formation in the middle Jurassic became the most important reservoir of oil and gas with its high porosity and permeability in Turpan Depression.