The purpose of this paper is to improve the dynamic stability of power systems equipped with offshore wind farms and HVDC transmission lines. Since wind farms are affected by environmental factors and cannot have a constant production capacity, the effect of wind turbine and HVDC system on power oscillation mode is investigated and a suitable solution for selecting input-output signals and stabilizing complementary controller design is proposed. In the proposed method, using the concepts of controllability, observability and decomposition of single values, the best path for the design of the complementary controller is selected among the input-output signals, then the stabilizer controller is designed based on neural networks and to improve frequency Stability-Voltage is used. The simulation results show that the proposed controller performs better than the classical controllers in terms of response speed, settling time, and voltage fluctuations in the presence of disturbances and confirms the performance of the selected control system.