Abstract: It is difficult to build a precise mathematic model and implement real-time fault diagnosis for the starting process of large liquid propellant rocket engines.According to its characteristics of the starting process,a fault diagnosis model based on Petri net with timing constraints and a fault diagnosis algorithm for the starting process of liquid propellant are proposed and implemented.Its Simulink simulation model and GUI simulation interface are built in MATLAB.Simulation experiments show that the proposed model and algorithm are valid and effective.