The application of servo cylinder is increasing in engineering test field. The impact of large volume cylinder during unloading is too large, which may affect the normal test. In order to solve this problem, the front and back chamber oil volume produced by unloading was calculated through unloading principle and liquid compressible theory. The impact was reduced by using proportional valve to control the unloading rate of front and back chamber. What’s more, the results with or without proportional valve were compared through AMESim simulation and actual test respectively. It is concluded this method can reduce the unloading impact of large volume cylinder.