Taking a high pressure piston pump with a pre-compression chamber as the object, the internal three dimensional fluid model was built and simulated by computational fluid dynamic software PUMPLINX. The influence of load pressure, rotating speed and structure parameters of different pre-compression chamber on the flow ripple rate of high pressure piston pump outlet was analyzed. The simulation results show that when the pre-compression chamber damping hole diameter is respectively 2,3 and 4 mm, the pump outlet flow ripple rate is respectively 39.87%, 16.43% and 17.67%; when the pre-compression chamber damping hole span is respectively 5°, 9° and 12°, the pump outlet flow ripple rate is respectively 17.56%,13.21% and 14.15%; when the pre-compression chamber volume increases from 200 cm3 to 300 cm3, the pump outlet flow ripple rate reduces from 2267% to 1441%, but with the pre-compression chamber volume increases from 300 cm3 to 400 cm3, the pump outlet flow ripple rate remains basically unchanged. The simulation results lay theoretical foundation for the design and optimization of the pre-compression chamber structure in the pump. Finally, the flow test experiment of the high pressure piston pump was carried out, and the experimental results were consistent with the simulation results, which proved the correctness of the simulation.