The invention discloses a quick current-limiting protection circuit of a negative electrode modulator. The quick current-limiting protection circuit comprises a current sampling circuit and a gate voltage clamping circuit; the 1 end of the current sampling circuit is connected with the input end of the modulator and the 2 end of the gate voltage clamping circuit; the 2 end of the current samplingcircuit is connected with the negative end of a negative electrode high-voltage power supply and the 3 end of the gate voltage clamping circuit; and the 1 end of the gate voltage clamping circuit is connected with a gate of an internal switching tube of the modulator. Compared with the prior art, the quick current-limiting protection circuit has the following advantages: by virtue of the current-limiting protection circuit, the gate voltage of the internal switching tube of the negative electrode modulator is controlled, so that quick and effective current-limiting protection on the modulatorcan be performed; and a simple circuit, high current-limiting response speed and high reliability are achieved, so that it is ensured that the negative electrode modulator can perform current limitingwithout being damaged when the modulator suffers from sparking and overcurrent.