A sample introduction device comprises a sampling unit, a gas suction pump, adsorption units, a piston cylinder and a desorption cylinder that comprises a desorption chamber, a carrier-gas inlet, a split/purge vent and an analyzer nozzle communicating with the desorption chamber. A heating film and a temperature sensor are provided on outer wall of the desorption cylinder. The piston cylinder above the desorption cylinder comprises two piston chambers, each of which is provided with the adsorption unit and in communication with the desorption chamber. The piston cylinder comprises a sample-gas inlet connected to the sampling unit and a gas-suction-pump orifice connected to the gas suction pump, each of which can communicate with both piston chambers. Each adsorption unit comprises an adsorption cylinder-like screen for holding adsorbents and a piston rod slidably mounted in the piston chamber. Each adsorption cylinder-like screen can simultaneously communicate with the sample-gas inlet and gas-suction-pump orifice.