The invention provides a sample injection device for a trace detector. The sample injection device comprises a sample injection chamber arranged in the sample injection device and used for desorbing a sample from a sample injection element, and a valve assembly used for communicating the sample injection chamber with a fluid in the transfer tube of the trace detector during sample injection. According to the sample injection device provided by the invention, with the adoption of the structure, for example, the sensitivity of a detection device can be improved by increasing the transmittance of the sample; meanwhile, isolation between the interior environment of the transfer tube and the external environment is realized, pollution to a transfer area is avoided, and the invariability of important parameters such as the sensitivity, substance peak position and resolution of the instrument can be maintained, thus realizing the stability and consistency of the instrument.