The theoretical and numerical simulation analyses for the pressure variation in the vacuum chamber that simulated high altitude condition during ignition transient of reversely-fixed thrust chamber were performed.Results shows significant pressure increase in the chamber in the working process of rocket nozzle,which is mainly caused by the kinetic energy of high-speed exhaust gas.In order to ensure the vacuum degree in the chamber,a deflector is needed to guide the gas out of the chamber.The numerical analysis and experimental test results show that the pressure rise can be reduced by the configuration of gas deflector tube at exit of the thrust chamber.It indicates that this method is benefit to the vacuum of the test chamber.This research may provide a guide for the test of the attitude and orbit control system in the high altitude simulation test.