Taking the stretching machine main hydraulic system as an example, the proportional pump control hydraulic cylinder system was divided into two series of single inputsingle output device: proportional pump and hydraulic cylinder, the natural frequencies of the two devices were calculated respectively, then according to the relationship between the natural frequency of series systems, the natural frequency of the proportional pump control hydraulic cylinder system was calculated. The calculation result shows that the natural frequency of the hydraulic cylinder is the main part of the natural frequency of the proportional pump control hydraulic cylinder system, the natural frequency of the proportional pump has certain influence on the natural frequency of the whole system, especially when the initial equivalent volume is small.