The invention relates to a grid modulator, which is installed in to a high voltage generator and includes a voltage controlling circuit, an isolating boosting transformer, a voltage doubling rectifying circuit, a drive pulse generation circuit, an isolating drive transformer, an inverter circuit, and a sampling circuit. The voltage controlling circuit automatically adjusts and outputs a voltage value to the isolating boosting transformer according to a set voltage value; the primary winding of the isolating boosting transformer is connected with the output end of the voltage controlling circuit to boost the voltage outputted by the voltage controlling circuit; the voltage doubling rectifying circuit implements doubling rectifying on the voltage boosted by the isolating boosting transformer; the drive pulse generation circuit receives the command of the control part of the high voltage generator and outputs a drive pulse; the primary winding of the isolating drive transformer is connected with the output end of the drive pulse generation circuit; the secondary winding of the isolating drive transformer is connected with the inverter circuit; the inverter circuit inverts a DC voltage formed by the voltage doubling rectifying circuit; the sampling circuit samples a voltage signal and respectively feeds back the voltage signal to the voltage controlling circuit and the control part of the high voltage generator. The grid modulator has simple structure and self-cutting function, and can meet insulating requirements.