A nail gun with an improved nail-belt transmission mechanism that comprises a ratchet wheel coupled to a transmission gear wheel through a transmission axle. One end of the transmission axle is provided with a link capable of driving the first ratchet wheel by a second ratchet wheel mounted on the inner wall aside the transmission gear wheel. There is a retaining part engaged with the second ratchet wheel from above for maintaining equal-pace rotation of the transmission gear wheel, thereby facilitating screw nail mounting by the nail gun.