The invention discloses a gun reamer which comprises a reamer body which is formed by sequentially arranging a reamer head, a neck portion and a handle portion in a front-and-rear mode. A blade is arranged on the reamer head, and the reamer body is provided with a center water injection hole which sequentially penetrates through the reamer head, the neck portion and the handle portion. A plug block is arranged in the front end of the center water injection hole, and water outlets are sequentially and axially arranged from the reamer head to the neck portion at intervals. One ends of the water outlets are communicated with the center water injection hole, and the other ends of the water outlets are led to the surface of an outer circle of the reamer head or the surface of the outer circle of the neck portion. The gun reamer is characterized in that the water outlets are arranged by surrounding the circumference of the center water injection hole simultaneously at intervals. According to the gun reamer, the surface smoothness of a machined hole can be enhanced.