The present invention relates to a method for a multi-system core network notifying a key to a multi-system radio access network, and a multi-system network. The method includes: the multi-system core network sending a key of a source system and key indication information to the multi-system radio access network; the multi-system radio access network receiving the key of the source system and key indication information; and when a user equipment performs multi-system handover, if the key indication information indicates that the key of the source system is an original value, the multi-system radio access network performing key conversion on the key of the source system to obtain a key of a target system. With the method and multi-system network of the present invention, the interruption time of user plane data can be reduced during the multi-system handover.
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