Embodiments of the present invention disclose a multi-network access control method, a communication system, and relevant devices to prevent interruption of normal network communication when the multi-access function is used. The method includes the following steps: A User Equipment (UE) receives multi-access indication information sent by a network device; the UE judges, according to the multi-access indication information, whether the UE uses a multi-access function; and the UE initiates an access connectivity establishment from multi-networks if the multi-access function is used. The embodiments of the present invention further provide a communication system and relevant devices. The embodiments of the present invention may prevent interruption of normal network communication when the multi-access function is used.