A detachable driving assembly for a toy gun has a first casing containing a piston device and a second casing containing a gear set. The first casing and the second casing are detachably combined by a pin and a levering lock. The piston device has a piston body with a spring and a toothed rack formed under the piston body. The gear set mainly comprises a miter gear, a reduction gear and a half-toothed gear sequentially rotated by a motor to drive the piston device. The half-toothed gear has a periphery, multiple teeth partially formed on the periphery, and an idle area formed on a remainder of the periphery. Thereby, when the teeth engage with the toothed rack to run with the half-toothed gear, the piston device is driven to a compressed condition. When the half-toothed gear enters the idle area, the piston device is released to punch a projectile.