A nail gun includes: a shell, a cylinder, a piston, a gear frame, and a stress relaxation device. The cylinder is disposed in the shell. The piston is disposed in the cylinder and includes a piston rod and a piston disc. The gear frame is coated on a front section of a cylinder exterior. The stress relaxation device is disposed in the cylinder. The stress relaxation device includes a first stress relaxation ring, a second stress relation ring and a stress relaxation unit. The first stress relaxation ring and the second stress relaxation ring are both connected with the stress relaxation unit and not in relative movement to the stress relaxation unit.