Abstract: A physical model of the automatic control system of a pump-fed Liquid Rocket Engine (LRE) is constructed, for which a nonlinear mathematical model is built. Considering the interfering modalities probably happening and input with step form, the dynamic characteristic in each interfering status is simulated by using the small deviation calculating method. Comparison between the computed results and the test data shows that: when the interfering signals are varied within small bound, the mathematical model is relatively accurate and the calculating method can meet the simulating requirement. This work has signifcance for optimizing control parameters and reasonably adopting control system.