The invention discloses a gaseous chlorine dioxide generator which comprises a preliminary mixer, a generator main body and a residual liquid treatment tank, wherein a material inlet is formed in one end of the preliminary mixer, and the other end of the preliminary mixer extends into the generator main body; the residual liquid treatment tank comprises a first-stage residual liquid treatment tank and a second-stage residual liquid treatment tank; the generator main body is located in the first-stage residual liquid treatment tank; a first-stage gas stirring pipe, a second-stage gas stirring pipe and a third-stage gas stirring pipe are respectively arranged in the generator main body, the first-stage residual liquid treatment tank and the second-stage residual liquid treatment tank. The gaseous chlorine dioxide generator has the technical effects that by the adoption of four stages of reaction and three stages of gas stirring, due to the preliminary mixer, raw materials can be fully mixed and subjected to preliminary reaction, and then enter the generator main body for further reaction and gas stirring, so that gaseous chlorine dioxide can be fully discharged; the gaseous chlorine dioxide enters the first-stage residual liquid treatment tank and the second-stage residual liquid treatment tank in sequence for gas stirring reaction; due to the various stages of reaction and gas stirring, the final reaction rate can be 90 percent or above.