Based on the grid-connected PV system and the parallel APF have many similarities on topological structure, operation mode and control method, we put forward a new kind of grid-connected PV integration system, which can combine active filter with compensation of reactive power. In order to detect the harmonics and reactive current of the power supply system, we propose a detection method based on FBD. Compared with the current detection method based on the theory of instantaneous reactive power, this method has no complex Park transform and d-q transform, so it can detect the real-time current more effectively. In the meanwhile, it can be flexibly applied in unbalanced three-phase power supply system. The simulation results have shown the correctness and effectiveness of the current detection method.