In distributed generation systems, constant power (PQ) control is often used in order to transmit energy from wind energy and photovoltaics to the micro-grid unit as efficiently as possible. When the micro-grid system is operating with a non-linear load, in order to ensure the quality of the incoming network current, the harmonic suppression function is added to the traditional PQ control strategy, that is, the PQ control and the active power filter (APF) function are combined into one. A multi-function grid-connected inverter with APF function is formed, which not only transmits active power to the grid, but also achieves the purpose of compensating for harmonics. This paper starts with the grid-connected inverter and analyzes its working principle and PQ control strategy. Based on this, the harmonic suppression function is added and explained in detail, mainly including harmonic detection based on instantaneous reactive power theory, and multi-resonant controller section. Finally, simulation and experiment verify the correctness and feasibility of this control strategy.