The invention relates to the technical field of medical instruments and discloses a surgical instrument. The surgical instrument comprises a nail anvil arm and a nail box arm, the nail box arm is provided with a nail box, the nail anvil arm is provided with a nail anvil and a gap adjusting mechanism used for adjusting the closed gap between the nail anvil and the nail box, and the near end of the nail anvil arm is connected with the nail box arm through a rotary shaft. The gap adjusting mechanism comprises a transmission part and a driving mechanism, wherein the transmission part can move relative to the nail anvil arm in the direction perpendicular to the axis of the rotary shaft and perpendicular to the length direction of the nail anvil arm, and when the nail anvil and the nail box are closed, the transmission part abuts against the gap adjusting plane; the driving mechanism is in transmission connection with the transmission part and used for driving the transmission part. In the surgical instrument, the stability of the size of the closed gap between the nail anvil and the nail box is high, and accordingly the stability of mechanical properties of the surgical instrument is improved.