A variety of topology structures and control strategies of active power filter (APF) are analyzed.Based on the four basic control strategies,load harmonic current control,grid harmonic current control,filter line harmonic current control and load harmonic voltage control,the steady state and dynamic state analysis of typical APF structures such as shunt hybrid APF,series hybrid APF and APF series with PF connecting to the grid are carried out.The experimental waveforms,which verifies the correctness of the theory analysis,of APF working on steady state and dynamic state are also given by an application example.The conclusions obtained in this paper is of great significance in structural selection and practical application of APF.