Abstract: The harmonic exceeding phenomenon may be triggered when offshore wind farms are merged into the power grid via AC submarine cables under the influence of the background harmonic, which seriously threatens the safe and reliable operation of the power system. However, the static synchronous compensator(STATCOM) under traditional control strategies have poor mitigation performance and tripping risk when mitigating harmonics. To tackle this issue, a harmonic mitigation method based on quasi resonant sliding mode perturbation observer(QRSMPO) compensation strategy is proposed in this paper, to improve harmonic mitigation ability and operational stability of STATCOM. First, the propagation process of the harmonics on STATCOM is analyzed, and the feasibility of introducing an observer in the STATCOM control loop to achieve harmonic mitigation is determined. Then, the advantage of using QRSMPO to estimate and compensate for harmonic disturbances in real time, the harmonic mitigation strategy based on QRSMPO compensation is designed from the perspective of blocking harmonic propagation. Furthermore, the transfer function of QRSMPO compensation strategy is derived, which proves the effectiveness and stability of the proposed strategy. Finally, taking Rudong offshore wind farm as the research object, the mitigation performance of the QRSMPO is evaluated. Compared with the existing methods, the proposed strategy can effectively mitigate harmonic in the offshore wind farms under various operating conditions, and presents stronger stability and robustness.