A method for realizing an automatic protection switching of a transmission device is provided, and the method includes that: according to a received automatic protection switching trigger condition and information of each line sub-card, which are transmitted by a cross sub-card via a Time Division Multiplexing Fabric to Framer Interface (TFI5) frame, a control sub-card determines to execute protection switching, and sends a protection switching command to the cross sub-card; and the cross sub-card completes the protection switching action. A system for realizing an automatic protection switching of a transmission device is also provided. According to the technical solution of the present invention, the automatic protection switching of the transmission device in an Optical Transport Network (OTN) is achieved conveniently.