The invention relates to an interface method of a power transmission and distribution reactive power optimization system. The interface system comprises the following steps: (1) building a transceiving order table; (2) collecting and sending data; (3) transforming and receiving data; (4) carrying out an idle optimization strategy; and (5) feedback. After a capacitor bank is switched to a distribution network idle optimization master station system, the variation is transformed to an in-station AVC (Automatic Voltage Control) system through a power grid. The above transform mode: (1) to (4) steps, is repeated, an AVC system adjusts a control strategy of devices in the station according to a new power grid real time signal, and controls through an AVC control collector The interface system and the interface method have the advantages that the structure is simple, the equipment cost is relatively low, the piggybacking is easily realized, the voltage quality is improved, and the electric equipment operation is safe and reliable.