Abstract: APF(active power filter) is one of effective solutions to harmonics in power system. In order to meet the real time and accuracy requirement of this way, an i_p,i_q harmonics detection method based on the instantaneous reactive power theory has been introduced by means of TMS320F240DSP. At first, the data and wave of input current and the current harmonics in 3 phases system have been obtaind through simulation of MATLAB on the i_p,i_q harmonics detection method. Then thedetailed implement on TMS320F240DSP has been completed with DSP developing tool-code composer. At last the result comparison has been done between MATLAB simulation and DSP calculation. The analysis and test result show that this method has the characteristics of real time and accuracy, and can meet the harmonics detection requirement of APF.