A simple model of propellant tank was established by neglecting mass and heat transfer between the propellant and pressurant.Then by employing the modularization modeling and simulation software for liquid rocket engine（LRE） test-bed gas system（LRETBMMSS-GS）

blowdown of a tank and pressurization of a LO2 tank pressurization system during engine firing were simulated.The simulation results are in good agreement with the analytical solution and test data.Accordingly

the software is validated to be effective and versatile.The progress of modeling two systems shows that the method of MMS is suitable for modeling complicated LRE system and can be used to simulate all kinds of working processes of LRE system.The simulation of LO2 tank pressurization system indicates that PID control parameters should be set reasonably and the initial opening of pneumatic diaphragm control valve should be adjusted to nominal pressurant mass rate

which is effective to improve stability of pressurization starting transient.