The present invention relates to a dopant gas generating device for supplying the dopant gas to the ion mobility spectrometry instrument, comprising: a doping container; an air inlet having an inlet end connecting with an upstream side of a carrier gas passage and an outlet end connecting with the doping container; an air outlet having an entrance end connecting with the doping container and an exit end connecting with an downstream side of the carrier gas passage; a dopant gas generating unit for releasing the dopant gas, wherein the dopant gas generating unit is disposed within the doping container. Through disposing the dopant gas generating unit, which is used for releasing the dopant gas, within the doping container, the dopant gas in the present invention is applicable with not only a solid state dopant, but also a liquid state dopant.