Embodiments of the present invention relate a method, an apparatus, and a system for assigning a tributary port number. The method includes: receiving a service path request message which is sent by a neighboring node and carries a service type; determining, according to the service path request message, an OTU link used to bear the ODU service, determining and reserving, according to the service type in the service path request message, a free tributary slot resource used to bear the ODU service in the OTU link, and assigning a tributary port number to the free tributary slot resource; and sending a service path acknowledgment message to the neighboring node. In the method, apparatus, and system for assigning a tributary port number in the embodiments of the present invention, the tributary port number is obtained automatically instead of being assigned manually through a network management system, and therefore, the efficiency of assigning the tributary port number is improved. Because the manual operation is avoided, the error rate of assigning the tributary port number is reduced.