Dynamic Voltage Compensator (DVR) can be used to handle the dynamic power quality problems in distribution network, such as voltage sags and voltage unbalance, and the parameters of the output filter will have important affect on the voltage sag compensation results. In this paper, the parameter design method of the output filter of a cascaded H-bridge based DVR applied in medium- voltage distribution grid is studied. Through the analysis of the distribution of inverter output voltage in the capacitor and inductor, and derives the relation between the value of inductance , capacitance and the voltage of inductance, capacitor, considering the relation of the low pass filter resonance frequency ,DVR equivalent switch frequency and system frequency, the calculates the value of capacitance and inductance. Combined with an examples, specific implementation steps are given, the correctness and validity of the design method of filter parameters proposed in this paper are verified by simulation.