Abstract: Aimed at improving the power capture capability of the direct-drive wave power system under irregular excitation force conditions,the maximum power extraction conditions were given based on the variational method,the corresponding perfect electromagnetic force was obtained by calculating it. In order to predict the unknown excitation force input,a sliding mode observer was designed based on the motor-hydrodynamic model,the system stability was proved by Lyapunov function. Taken the optimal electromagnetic force as the reference input,the desired current signal of the space vector control strategy tracking system is used to achieve the maximum power output. The results show that the proposed scheme has good robustness against model mismatch,higher prediction accuracy and output power of the system. It can detect the excitation force online,and promote the wave energy conversion rate of the system in complex wave conditions.