Veracity and real-time requirements for the harmonic detection are the most important factors to influence the active power filter. This paper adopts an exponential average low pass filter and a cascade form to effectively solve the conflict between the veracity of harmonic detection and real-time requirements and the finite word length of DSP design. A DSP based digital low pass filter is then presented for high performance harmonic detection in a series hybrid active power filter. The experiment results validate the theoretical analysis.