The differential pump driven by Fourier non-circular gears has obvious periodic impact phenomenon under load condition, but it does not exist under no-load condition. In order to reduce the impact under load conditions and improve the stability of differential pump, a numerical calculation model of differential pump was established, and the fluid-solid coupling calculation of flow field and driving non-circular gear of differential pump was carried out by numerical calculation method. The calculation results showed that there was a sudden change in the impeller’s torque at the instant of alternating suction and discharge conditions of differential pump. The main reason was the water hammer effect formed when the blade rotated to the inlet and outlet and closed or opened. In order to reduce the impact, microrounded edges of differential pump blades were processed to form a flow field transition zone. The simulation results showed that the periodic torque mutation of the input shaft of the optimized blades was reduced by at least 21.58%, and the pressure distribution of the suction and drainage chambers was more uniform. By verification, it was confirmed that the edge of the differential pump blade had a great influence on the torque characteristics. The maximum variation of the two impellers’ torque was decreased by 51.20% on average. And the optimization of the edge roundness had a great effect on reducing the water hammer effect and improving the impeller torque characteristics.