Abstract: With the increasing grid-connected capacity of offshore medium-and long-distance offshore wind farms, a large number of submarine cables are widely used. This leads to the problem of harmonic voltage and harmonic current exceeding the limit after offshore wind farms are connected to the system. Therefore, this paper analyzes the reasons for the excessive harmonics in the onshore centralized control station of offshore wind farms, and theoretically analyzes the design scheme of the active filter of offshore wind farms, and draws the conclusion that the harmonic current and harmonic voltage can meet the requirements of the national standard under comprehensive treatment. Finally, the high-voltage active filter is applied to Daishan and Shengsi wind farms. Through systematic tests, it is proved that it can effectively reduce the harmonic current and voltage of the grid-connected point, and verify the theoretical results.