An active filter based on a modular structure and used in the technical fields of electric power and electronics comprises a main active filter and a plurality of slave active filters, wherein the main active filter is shorted with the take-off three phases of the slave active filters respectively and is connected with a power network through a bus; and the main active filter is connected to the signal ends of the slave active filters through an optical fiber. The active filter has flexibility and convenience in the aspects of production, transport, installation, capacity expansion, and maintenance.