With the proposal of the "double carbon" goal, the structure of the new power system with new energy as the main body has changed significantly, especially the access of large capacity offshore wind power units, which has brought harmonic and other power quality problems to the grid. In order to study the characteristics of low frequency harmonics generated by fan connection, firstly, a theoretical model of low frequency harmonic content in the gridconnected current of fan is established. Then, an offshore wind power simulation model is built on the simulation software ETAP to verify the output harmonic characteristics of the wind farm under different output conditions. Finally, based on the output harmonic characteristics of the wind farm, the variation coefficient synthetic weighting method is proposed to optimize the configuration of the active power filter (APF) in the wind farm to improve the control effect of the harmonic in the wind farm, and the effectiveness of the method is verified by simulation based on an actual example.