In the shale gas separation and sewage system, the separator removes the sand and sewage in the produced gas and releases the sand-carrying sewage to the blowdown pipeline. As an important throttling component in this system, the sleeve-type blowdown valve is severely eroded during operation. To address the problem, this paper carries out numerical research on the erosion characteristics during the real opening and closing process of the sleeve-type blowdown valve based on the FLUENT fluid simulation software. The change of the erosion at different velocities, sand mass flow, sand size, and shape coefficient is analyzed, and the main factor affecting the erosion characteristics is evaluated. Based on the above research, a novel blowdown valve is proposed, and the erosion characteristics and flow performance before and after improvement are compared. The results show that when the velocity is 8 m/s, the maximum erosion rate after improvement is 4.79×10−5 kg/(m2·s). Compared with the maximum erosion rate before improvement of 1.22×10−2 kg/(m2·s), the erosion inhibition rate reaches 99.6%. Moreover, the pressure loss is obviously reduced after the improvement, and the flow capacity is enhanced. This provides useful guidance for the improvement of blowdown valves and ensures the safe operation of shale gas production.