Because the basic structure of the photovoltaic grid-connected inverter is the same as the active power filter, a unified control of the photovoltaic grid-connected inverter and the active filter can be realised. At present, the unified control system is mainly based on the detection method of three-phase voltage balance when compensating for the side harmonics of the power grid. However, in actual working conditions, the three-phase voltage imbalance may occur from time to time. At this time, the detection error of the voltage phase angle is large， harmonic response slows down, the actual compensation will have a large delay, reducing the system's harmonic compensation speed, resulting in poor side-by-side harmonic filtering effect. Aiming at the problem of harmonic compensation speed reduction, an improved harmonic detection method is proposed in this paper. When the three-phase grid voltage shows unbalanced phenomena such as phase shift and amplitude variation, the positive and negative sequence voltages are park-transformed at the same time. Through the current average module to get the fundamental wave voltage, and then accurately calculate the positive phase voltage phase angle, to ensure the speed of harmonic detection to improve the harmonic dynamic compensation effect. The simulative study of the improved harmonic detection algorithm is carried out by building a simulation model of a PV grid-connected and active power filtering unified control system in MATLAB/Simulink. The simulation results show that the improved harmonic detection algorithm can accurately extract the voltage phase ， perform harmonic compensation quickly and increase the compensation speed of the entire system by 0.057s when the three-phase voltage is unbalanced.