In the field of optical network communication technologies, a method, a device, and a system for transmitting a constant rate data stream are provided. The method for transmitting the constant rate data stream by the first network device includes: receiving a constant rate data stream; calculating an input rate of the constant rate data stream or a rate difference between the input rate and a standard rate of the constant rate data stream in each cycle of a reference clock; encapsulating the constant rate data stream into a Gigabit-Passive Optical Network (GPON) Encapsulation Method (GEM) frame by using a bit as a minimum encapsulation unit; and encapsulating the GEM frame into a GPON Transmission Convergence Layer (GTC) frame, and sending the GTC frame to the second network device through a GPON network. Therefore, the technical solutions may be widely applied to the GPON network.