A detergent/water mixing system for a water spray gun includes a gun body having a water inlet, a water outlet, a cylindrical transverse through hole between the water inlet and the water outlet, and a bottom axle hole perpendicularly extended from the transverse through hole, a detergent container detachably attached to the gun body at the bottom side, a nozzle jet mounted in the axle hole on the gun body and controlled to eject water into the detergent container and to guide detergent solution from the detergent container into the water outlet of the gun body, and a control valve turned by hand through a handle attached thereto in the transverse through hole on the gun body to control the flowing direction of water.