The disclosure provides a clamp circuit and a method for clamping voltage. The clamp circuit includes: a first switch control unit, connected with the high-potential terminal of the first stage output of a comparator and configured to clamp the voltage of the high-potential terminal to VGate1 when the voltage of the high-potential terminal is lower than a first pre-set value V1, and a second switch control unit, connected to the low-potential terminal of the first stage output of the comparator and configured to clamp the voltage of the low-potential terminal to VGate2 when the voltage of the low-potential terminal is higher than a second pre-set value V2, wherein the voltages of the first stage output of the comparator are between VGND and VCC. By the disclosure, the output voltage swings of the first stage of the comparator are limited, and thereby the processing speed of the comparator is improved.