The invention relates to a quantitative mixing device for a water treating agent. The quantitative mixing device comprises a fixed supporting seat, a conveying unit, a feed bin, a weighing unit, a primary mixing drum and a fine stirring drum, wherein the end surface of the fixed supporting seat is provided with weighing part supports used for supporting the conveying unit and fine stirring drum supporting columns used for supporting the fine stirring drum; the conveying unit is fixedly connected with and arranged on the end surfaces of the weighing part supports through bolts; two ends of theconveying unit are respectively provided with a rotating wheel; and the positions of the rotating wheels located at one ends of the conveying unit are respectively provided with a conveyor belt motorused for driving the rotating wheels to rotate. Thus, the quantitative mixing device provided by the invention has the advantages of convenient use, accurate weighing and high mixing efficiency.