With the gradual improvement of the intelligent level of the launch vehicle, in order to realize the load monitoring, fault diagnosis and disposal of the flight status of the launch vehicle, it is necessary to carry out fast and real-time calculation of the dangerous section load, that is, the lightweight calculation of the load. First, by combining theoretical formulas and proxy models, a lightweight calculation method is proposed to improve computational efficiency while ensuring computational accuracy. Secondly, the calculation process is re-planned according to the calculation amount, and the sections are classified. Next, for aerodynamic and control loads, the load calculation formula is derived based on the d’Alembert principle to ensure the accuracy of the calculation, and the calculation amount is reduced by pre-calculating the constant-value parameters related to the configuration. Lastly, for the sloshing load, a polynomial proxy model is built based on the simulation data, and the sloshing load is predicted through the proxy model to further reduce the calculation amount. The research results indicate that the proposed lightweight method can significantly improve computational efficiency. By combining the load variation patterns of different flight time periods and selecting the number of calculation sections reasonably, the calculation time can be controlled at the millisecond level. Compared with the finite element method, the calculation deviation of different types of sections is less than 5%. This method has important application value for multidisciplinary simulation and real-time calculation of flight load.