For gas generators based on pyrotechnic technology, as the structure becomes more complex, the internal ballistic characteristic is no longer just related to the flow field generated by propellant combustion. The fluid-structure interaction in the system plays an important role. In order to accurately predict the internal ballistic characteristic of the gas generator, a mathematical model of coupling the flow field generated by propellant combustion and the structural evolution in the system is established. The interior ballistic model is utilized to calculate the flow field generated by propellant combustion, and the finite element method is applied to simulate the structural evolution. The flow field and the structural evolution are coupled through a user subroutine interface in ABAQUS. The accuracy of the coupled model is verified by experiment, and the precision of the traditional classical interior ballistic model is compared with that of the coupled model. The results show that the pressure prediction error of the traditional classical interior ballistic model is more than 46% for gas generators with complex structures. While the coupling model is in great agreement with the experiment, and the error of pressure prediction is less than 5.0%.