Abstract: Aiming at the problems of increased fatigue damage and shortened service life of wind turbine main shafts caused by alternating loads and impact loads under complex wind conditions, an active support control strategy for main shaft loads based on Active Disturbance Rejection Control (ADRC) and Particle Swarm Optimization (PSO) is proposed. Firstly, combined with the lumped mass method and finite element analysis, the dynamic model of the wind turbine main shaft-bearing system is established, the transmission path and dynamic characteristics of the main shaft load are clarified, and the influence of multi-source excitations such as wind shear, tower shadow effect and grid disturbance on the main shaft load is quantified. Secondly, the architecture of the active support control system is designed. The Extended State Observer (ESO) is used to estimate the dynamic load and unmodeled disturbance of the main shaft in real time, the PSO algorithm is used to self-tune the parameters of the PI controller, and the nonlinear state error feedback compensation is combined to realize the precise suppression and active support of the main shaft load. Finally, a simulation platform is built based on MATLAB/Simulink, and combined with the actual operation data of a 3MW wind turbine, the proposed strategy is compared and verified with the traditional PID control strategy. The simulation results show that the proposed control strategy can effectively reduce the fluctuation amplitude of the main shaft radial load by 32.7% and the axial load by 29.5%, reduce the accumulation of main shaft fatigue damage, improve the operational stability of the unit, and has stronger robustness under complex working conditions such as sudden wind speed changes and grid frequency fluctuations. Keywords: wind turbine; main shaft load; active support; active disturbance rejection control; particle swarm optimization; fatigue damage