The invention relates to a clamp, in particular to a tool for machining the inner surface of a nozzle for a high-voltage circuit breaker to solve the problem that in the prior art, a clamp for machining the inner surface of a nozzle cannot position a blank of the nozzle firmly. The tool specially used for machining a nozzle clamping groove with a notch is designed for solving the technical problem and comprises a hollow sleeve body and a seal plate fixedly connected to one end of the sleeve body. Protruding blocks matched with the notch in the blank of the nozzle are designed on the end face, facing the sleeve body, of the seal plate. On one hand, the circumferential rotation of the blank of the nozzle relative to the tool and the movement towards the seal plate are limited, and on the other hand, the blank of the nozzle can be directly inserted into the sleeve body to achieve fast installing and positioning of the blank of the nozzle. Disengaging-preventing grooves for clamping jaws to be embedded therein are formed in the tool, and the clamping jaws are embedded in the disengaging-preventing grooves and clamp the end, without the notch, of the blank of the nozzle with little force so as to firmly position the blank of the nozzle.