Describs harmonic sources and influence of harmonics on the power system and electrical equipments, analyzes the working principle and characteristics of passive/active filters, and puts forward appropriate allocation and application of filters to avoid excess investment, meaningless energy loss and the high cost of maintenance due to abuse of active filters. In view of the specific circumstances of public buildings, analysis is limited to 380/220 V systems, and assumes that the non-linear parameters of power source can be ignored.