A data transmission method, a related device, and a related system. The method includes: receiving, by a first access network device, a data packet (for example, small data) sent by user equipment (for example, an IoT device), where the data packet includes a first cookie and raw data; verifying, by the first access network device, the first cookie, to obtain a verification result; and processing, by the first access network device, the raw data based on the verification result. Implementation of embodiments can reduce load on a network side when a large quantity of user equipments need to perform communication, thereby increasing data transmission efficiency.