At present, many detecting methods for harmonic and reactive currents have been put forward. Though having their characteristics respectively, they have insurmountable problems, such as vast calculating work, bad real time performance, low measurement precision and so on. Based on the foundation of load current analysis, a new real-time detecting method for harmonic current based on iterative algorithm in active power filter is proposed, It has the characteristics of very little calculating work, nice real time performance and high measurement precision. The correctness of the method is verified by theory analysis and computer simulation research.