A cap feeding device for a roofing gun includes a moving member and a pressing member which is pivotably connected to the moving member. Both of which are engaged with the rail unit of the container for receiving stack of caps. A torsion spring is biased between the two respective connection portions of the moving member and the pressing member. A coil spring has one end connected to the moving member and the other end of the coil spring is fixed to a bottom of the rail unit so that the moving member is movable along the rail unit. The caps can be directly pressed into the container and the user simply pulls the moving member upward till the pressing member pops outward to press on the top of the stack of the caps.