Embodiments of this application disclose an encoding method and a related device. The method includes: receiving a to-be-encoded code block whose length is L, where L is a positive integer; and encoding the to-be-encoded code block to obtain a forward error correction FEC code, where a valid information length K of the FEC code is an integer multiple of a largest prime factor of L, and a total length N of the FEC code is a sum of K and a product of 2 and an error correction capability T of the FEC code. According to the embodiments of this application, it can be ensured that an FEC codeword satisfies a requirement for a low latency and a high gain.