Abstract: Using the full-flow staged combustion cycle LH2/LOX engine as an example,the engine response characteristics were investigated.The non-linear dynamic mathematical model of the full-flow staged combustion cycle was presented.The variable step Runge-Kutta algorithm for solving the ordinary differential equations was developed.The immunity strategy algorithm and Runge-Kutta algorithm were combined to form this new algorithm.Applying the variable step Runge-Kutta algorithm,the transient performance of the full-flow staged combustion cycle was simulated.Calculating results show that variation of the propellant inlet pressure makes little impact on engine performance parameters,and the engine will transit to a new stable state.The parameter transition curves are smooth.When the engine thrust is adjusted in a small range within a short period,the parameter transition curves are also reposeful.But when the adjusting range becomes large,the parameter transition curves will fluctuate acutely.Thus,when large range adjusting is required,the time of adjusting should be lengthened or the adjusting should be divided into adequate steps.