Abstract: A voltage-source-converter based high voltage direct current (VSC-HVDC) system can provide auxiliary frequency support service to the receiving-end power systems due to its flexible power control capacity. A frequency auxiliary control based on a power disturbance observer is proposed for the grid-forming inverter station. According to the natural responses of the grid-forming converter to grid disturbance, a power-disturbance observer is designed for the receiving-end power system, which can estimate the system states and the power disturbance in real-time with limited measurements. With the estimates of disturbance and states, the state feedback controller and power disturbance compensator are designed according to the linear quadratic optimal control theory, which can achieve the accurate and flexible power regulation and frequency support. Two test systems are built in the PowerFactory-MATLAB co-simulation platform to verify the performance of the proposed control strategy. The results show that the proposed control strategy can secure the frequency stability of the receiving-end power system under the load variation.