Embodiments of the present invention disclose a communication method, a related device, and a system. The system may include a terminal, a first access network node (AN), and a second AN. The first AN is configured to determine that the terminal meets a condition of being handed over from the first AN to the second AN, where a value of a target parameter used for encryption and/or integrity protection when the terminal and the first AN communicate with each other before the terminal is handed over to the second AN is equal to a first reference value. In the system, the first AN may further be configured to send a target message to the second AN to instruct the second AN to obtain a second reference value. The second AN may be configured to obtain the second reference value based on the target message. Furthermore, the terminal may be configured to obtain the second reference value, where the second reference value is used as a value of the target parameter used for encryption and/or integrity protection when the second AN and the terminal communicate with each other. According to the embodiments of the present invention, security performance of the terminal can be improved.