Abstract: In order to quantitatively evaluate water blocking damage of tight reservoirs in flowback period, based on the relationship between capillary pressure and relative permeability with water saturation, Darcy's formula was used to characterize gas-water two-phase flow equations in invaded areas, and a quantitative evaluation model of water blocking damage of fracturing fluid in tight reservoir was established. The water saturation, relative permeability and permeability damage degree of invaded areas in flowback period were solved by the Taylor's nonlinear solution method, and the effect of fracturing fluid invasion depth, fracturing fluid viscosity, flowback and production differential pressure and pressure sensitivity were analyzed. The results show that, water blocking damage starts to gradually be relieved once the pressure differential of flowback and production exceeds the capillary force. The higher the differential pressure between flowback and production, the more effectively water blockage can be relieved and the less damage water blockage can cause. It is challenging to release the water blockage at the flowback period when the invasion depth, viscosity, and stress sensitivity coefficient of the fracturing fluid are high. The higher the damage degree of the water blockage, the lower the recovery of gas will finally be.