Abstract: In order to evaluate and optimize the reservoir protection effect of polysulfonated oil mixing drilling fluid used for deep fractured tight sandstone oil reservoir development of the northern Tarim Basin, the experiments were conducted on the dynamic and static damage of drilling fluids, the loading capacity of filter cakes, fluid sensitivity, water phase trap damage and fluid compatibility. Meanwhile, the optimization strategy of polysulfonated oil mixing drilling fluids was proposed after the analysis of the main damage mechanisms and discussion on the protective effect evaluation method of drilling fluids. The experiment shows that the dynamic damage degree of drilling fluids in the reservoir is medium strong-strong; the static damage degree of filtrate is medium strong-strong; the loading capacity of the fractures with slit width of 100 μm or less than 100 μm reaches 10 MPa; fluid sensitivity damage is weak; water phase trap damage is strong; the compatibility of filtrates and formation fluids is poor. In addition, water trap, incompatibility of fluids and solid phase invasion are the main ways by which drilling fluids damage the reservoir. Furthermore, poor sealing capacity of polysulfonated oil mixing drilling fluid for the fractures with large slit width in the reservoir and poor ability to reduce fluid loss for the fractures with small slit width are the main reasons for the reservoir damage. Comprehensive analysis of the main factors of drilling fluid damage, fully tapping the potential of the experimental data of dynamic damage, and evaluation of degradation ratio of solid phase abrasion particle size are needed for the reservoir protection effect evaluation of drilling fluids. Improvement of shielding temporary plugging property of drilling fluids and compatibility of drilling fluids and reservoir fluids, and adding surfactants will enhance the protective capacity of polysulfonated oil mixing drilling fluids for deep fractured tight sandstone oil reservoir.