Disclosed is an ion mobility spectrometer. The ion mobility spectrometer comprises a sample injector (14), a semipermeable membrane (15), an ionization region (16), a terminal electrode (18), an ion storage region, a drift region and a Faraday plate (22), which are arranged along a drift tube in turn, wherein one or more air inlets (29) connected with a needle valve (30) and a first filtering device (31) are arranged in one side near the semipermeable membrane (15) of the ionization region (16), there is at least one opening in the terminal electrode (18), whose diameter is smaller than the diameter of the air inlets (29), an air outlet (28) connected with an air extracting pump (27) is arranged near the terminal electrode (18) in the ionization region (16), the drift region is separated from the ionization region (16) by a micropore structure of the ion storage region, another air outlet (25) connected with another air extracting pump (26) is arranged at one side near the Faraday plate (22) of the terminal electrode (18).