The invention relates to circuit design for network devices of router and Ethernet exchange in communication technique. The disclosed bus exchange method and bus exchanger through MII management interface bus can read data in I2C serial interface bus. The method includes step: (1) unit connected to first bus sends command to bus exchanger for reading data in second unit connected to second bus; (2) bus exchanger parses the said command; based on parsed result, bus exchanger reads out data stored in unit connected to second bus, the data are stored; (3) unit connected to first bus determines whether operation that is to say bus exchanger reads data stored in unit connected to second bus is completed or not. If yes, the data are read out.