In light of the characteristics of coupling vibration in the wind blade fatigue loading process, a dynamic blade fatigue loading system model block driven by a rotating eccentric was established. An electromechanical coupling mathematic model was derived using the Lagrange equation. The system dynamics equations were approximate analysis by using the averaging method, then the torque balance equation of vibration was obtained. Based on analysis of the variation in vibration frequency, the simulation model was established. Numerical simulations on the system frequency capture process, and revealed the characteristics of a self synchronous vibration system. Finally, the correctness of the mathematical model and simulation results was verified by the wind blade fatigue loading. The results showed that the blade frequency and vibration amplitude fluctuation when the deviation between the driving and natural frequencies of the blades was obviously large. The frequency deviation in a small range (0.47~0.62 Hz), eccentric rotary system and blade will appear in frequency capture, and then amplitude tended to be stable, but decreased. When the load torque was larger and the motor power was insufficient, the eccentric block will rotate speed jump.