The invention discloses a sample injection device for an ion mobility spectrometer (IMS). The device comprises an inner sleeve component and an outer sleeve component, wherein an inner cavity is defined in the inner sleeve component; one end of the inner cavity is communicated with an inlet of a mobility tube via an inner channel and the other end thereof is provided with an inner end cover with an inner hole; the outer sleeve component is arranged into an eccentric sleeve which is coaxial with the inner sleeve component and can rotate relative to the inner sleeve component so as to form sleeve cavities between the inner and outer sleeve components; at least a communicating hole communicated with the inner channel selectively is arranged at one ends of the sleeve cavities and an outer end cover is arranged at the other ends of the sleeve cavities; a first outer hole selectively communicated with the inner hole and second outer holes communicated with the sleeve cavities are arranged on the outer end cover; and the outer end cover can rotate between a first position and a second position relative to the inner end cover so as to selectively introduce the samples to be tested into the inner channel via the inner cavity or the sleeve cavities. Besides, the invention also relates to a method for sampling solids and gases by using the sample injection device.