Abstract: An assembling structure for upper and lower housings of a water spray gun, wherein the water spray gun includes a body having the upper and the lower housing, and the assembling structure of upper and lower housings comprises the body including a bore and first and second locking holes, the upper housing includes a first engaging orifice correspond to the bore, a first bolt is used to screw the bore and the first engaging orifice to lock the upper housing to the body, the lower housing includes a second engaging orifice in response to a second locking hole of the body, a second bolt is used to screw the second engaging orifice and the second locking hole to lock the lower housing to the body, thereby assembling the upper and the lower housings together and fixing the first and the second bolts in the upper and the lower housings.