This paper uses longitudinal-lateral-torsional integrated beam model and coupled mass method to simulate load-bearing structure of rocket and liquid propellant, and also uses RBE2 element to simulate the joints between the core and the booster of launch vehicle. Different payload’s base frequency can be obtained by modifying the finite element model of payload. The author chooses four typical second states in the process of flying, and obtains the first ten natural frequency of the rocket in three different payload’s base frequency. Analysis shows that the integrated beam model is very simple and practical in analyzing the dynamic characteristics of rocket, and the results can meet the requirements. The model can be used for modal analysis at the early stage of design. At the same time, the effects of payload’s base frequency reduction on dynamic characteristics of rocket can be obtained by analyzing the natural frequency data. These conclusions can provide some basis to the designer of satellite.