The invention discloses a spacecraft sealed cabin gas sampling and analyzing system for a vacuum thermal test. The system comprises a space environment simulator, a sampling and analyzing system and agas sampling pipeline, a sealed cabin is arranged in the space environment simulator, the sampling and analyzing system comprises a sampling bin, a gas analyzing device, a gas detection device, a pump set and a valve group, and the sealed cabin is in communicated connection with the sampling bin through the gas sampling pipeline. The system is simple in structure, high in stability, capable of monitoring gas composition in the sealed cabin, few in monitoring error, high in accuracy, capable of avoiding subsequent risk of the sealed cabin and simple in monitoring process operation; when pressure of the sealed cabin is lower than 50Pa, vacuumizing pressure difference sampling is not needed, so that energy damage is lowered, diffusion to the sampling bin through thermal motion can be realized, and the gas composition in the sealed cabin can be directly analyzed through a spectral mass spectrometer.