Overload changes the internal ballistics of the motor and the erosion of the thermal protection structure, which increases the coupling degree between engine design and flight trajectory design. Conventional solid rocket overall-motor discrete design is difficult to fully consider this coupling relationship. By studying the coupling relationship between internal ballistics, external ballistics and thermal protection structure, an internal ballistics model and a thermal protection structure model considering flight overload are formed. An integrated simulation method based on internal ballistic model, external ballistics model and thermal protection structure model is established to achieve accurate prediction of internal, external ballistics and thermal protection structure erosion under overload. The calculation results show that under overload, the pressure of the motor increases, the altitude and the local flight path angle of the shutdown point increases, while the flight speed remains basically unchanged. The total mass of the thermal protection structure increases and the thermal protection structure thickness of the nozzle changes, requiring strengthened thermal protection. The integrated simulation correctly predicts the changes of internal and external ballistics under overload, laying the foundation for the joint design of internal ballistics, external ballistics and thermal protection structure, which can improve the integration degree of solid rocket overall-motor design.