Abstract: An optical power value transmission method and system, and a related device is disclosed. The method includes: A source network device obtains optical multiplexing section link information that indicates an optical fiber connection relationship between any two adjacent network devices between the source and a target network device of an optical multiplexing section which are located in different data communication networks. The source network device determines, based on the optical multiplexing section link information, a first output port that connected to a downstream network device by using a first optical fiber, and the downstream network device is connected to the source network device and that is indicated by the optical multiplexing section link information. The source network device obtains, on the first output port, a first output power value of an optical signal and sent it to the downstream network device by using the first optical fiber.