In practical industry fields, harmonic, negative sequence current and reactive current need to be restrained to meet the requirements of consumers. The active power filter is a new brand power electronics device. In the three-phase unbalanced circuit, it can restrain the negative sequence fundamental current except for restraining harmonic. In order to restrain harmonic and negative sequence fundamental current at the same time, it is required to detect the harmonic and negative sequence fundamental current accurately. In this paper, unbalanced three-phase circuit has been compensated based on the 66KVA active power filter. The quickness and accuracy in detecting negative sequence fundamental current and harmonic current in each order of APF is proved by simulation and experiments in this paper. After compensation, system current distortion factor has reduced from 28.5% to 2.79%, which meets the international standard of harmonic restraining.