The utility model provides an alternating current filter. The alternating current filter comprises a pure fiber current sampling sensing head and a plurality of action loops mutually and completely independent, each action loop of the plurality of action loops comprises a measuring optical fiber, a remote module connected to one end of the measuring optical fiber and used for sending optical fiber signals, a merging unit connected to the other end of the measuring optical fiber and used for receiving the optical fiber signals sent from the remote module, and a starting protection unit used for performing TDM protocol communication with the merging unit. The alternating current filter is advantaged by high measuring precision, good vibration resistance, and high reliability of protection action.