A rivet gun has a barrel, a shaft, a handle and a pneumatic cylinder assembly. The shaft is mounted under the barrel. The handle is mounted around the shaft. The pneumatic cylinder assembly is mounted under the handle and has a top cover, a fastener, an annular cylinder and a bottom base. The top cover, fastener, annular cylinder and bottom base are detachably assembled so can be replaced easily without changing the whole pneumatic cylinder assembly. The maintenance costs of the rivet gun are reduced.