A spray gun capable of mixing detergent includes a detergent tube having a water guiding tube passing through its upper cover, an interior tube fixedly connected to its bottom having a horizontal water inlet communicating with the detergent tube; a separate tube fixedly put between the interior tube and the adjustment tube for connection, an upper end of the separate tube connected to the water guiding tube, an inside of the separate tube partitioned by a partition wall to form an upper tube hole and a lower tube hole, an upper end of the upper tube hole having a detergent guiding inlet, a bottom of the upper tube hole having a water guiding inlet, the lower tube hole connected to a water source, an upper end of the lower tube hole having a water outlet; and an adjustment tube having an upper extension tube for putting the interior tub.