Abstract: We addressed the time evolution of pressure inside space-craft, originated from thermal outgassing of complicated components and insulation materials, in thermal test with the lab-built simulator to determine the safety margin of vacuum discharge.The influence of the pressure difference between the simulator and spacecraft was also investigated.First, the thermal outgassing characteristics were empirically modeled by data analysis of the four sets of measured results; second, the outgassing behavior was theoretically evaluated in molecular flow theory; and finally, the pressure margin of safety in the simulator of thermal test was determined.The calculated results show that the pressure difference has a major impact.To be specific, when it comes to vacuum discharge in thermal test, the safety margin should be a background pressure of 1×10^-5 Pa in the simulator and generally speaking, a pressure below 10^-4 Pa is a good safety tolerance.