The paper defined a concept of the utmost shapeable angle for the gas metal arc welding based rapid forming process, which is used to judge supporting material need to be employed or not when a inclined wall is deposited. If the angle formed by the outer normal of the inclined wall and building direction exceeds the sum of the utmost shapeable angle and π/2, then supporting material must be deposited to finish the inclined wall along the given building direction. The effects of metal transfer mode and welding speed on the utmost shapeable angle were studied. It is found that the ‘one droplet per pulse’ metal transfer mode in pulse-current welding based RP may obtain a bigger utmost shapeable angle than short circuit metal transfer mode does. The welding speed influences the utmost shapeable angle through heat input and bead profile. An abrupt bead is harmful to get a bigger utmost shapeable angle. Within a welding speed extension from 9 mm/s to 30 mm/s, the utmost shapeable angle gets the maximum value of 55° when the welding speed is 18 mm/s.