The invention provides a bypass circuit for a power unit of the preset voltage type high voltage inverters of a relay. The bypass circuit comprises a diode rectification bridge parallel connected beside an H inversion bridge, and a controlled silicon, and is characterized in that a preset voltage circuit is further arranged; the preset voltage circuit comprises at least one relay, at least one charge resistor, current-limiting resistor and charging capacitor; the relay and the current-limiting resistor are connected in parallel between DC positive busbar or negative busbar of the power unit and the controlled silicon; the charge resistor and the charging capacitor are connected in parallel beside the controlled silicon after being connected in series. In the bypass circuit with such structure, the voltages of two ends of the controlled silicon is preset to be close to the voltage value of the busbar under the condition that the bypass circuit is not provided to power unit; the diode rectification bridge and the controlled silicon in the bypass circuit no longer bear dv/dt exceeding endurance capacity thereof when the power unit operates; therefore the safety and the reliability ofthe inverter are ensured, and the fault of the power unit can be effectively bypassed.