The invention discloses a method for communicating a third-level domain and a second-level domain in a software-defined packet transport network (SPTN) and a system. The method comprises the following steps that a database collects external information; a primary control unit receives services, and performs path calculation according to a source address and a destination address of the services; and a secondary control unit issues a task after analyzing service parameters transmitted by the primary control unit. The system comprises the primary control unit and the secondary control unit, wherein the primary control unit comprises a task receiving module, a task analyzing module, a task issuing module, a link protection module, an alarm module, a primary database and a task uploading module; and the secondary control unit comprises a secondary task issuing module, a secondary database, a second-layer anti-looping module, an ARP module and a switch module. Through adoption of the method and the system, communications between the third-level domain and the second-level domain of an SPTN domain can be realized, namely, service flows can be transmitted from an Internet protocol (IP) layer to a media access control (MAC) layer and can be transmitted from the MAC layer to the IP layer. Moreover, link protection, alarm and quality of service (QoS) setting functions are provided, so that the reliability and robustness of the network are ensured.