The invention provides a bypass circuit for a power unit of the preset voltage type high voltage inverters of a transformer. The bypass circuit comprises a diode rectification bridge connected in parallel 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 a transformer, a rectifier, at least one relay, and at least one charge resistor, a current-limiting resistor and a charging capacitor; the AC output by the transformer is converted into DC by rectification of the rectifier; a DC filter capacitor is in bridge connection with the DC output terminal of the rectifier; the relay and the current-limiting resistor are connected in series between at least one terminal of the anode and the cathode of the DC output terminal of the rectifier and the controlled silicon; the charge resistor and the charging capacitor are connected in parallel beside the controlled silicon after being connected in series.