A method and apparatus are provided for managing session information of an access terminal (AT) that has moved from a source subnet to a target subnet in a mobile communication system for high-speed data transmission. The AT transmits a message with a prior universal access terminal identifier (UATI) to a target access network (AN) to perform session negotiation with the target AN of the target subnet. The target AN transmits a message with the prior UATI to a target packet control function (PCF) to request retrieval of session information of the AT. The target PCF sends a request for transmission of prior session information for the AT to a source PCF of the source subnet, found using the prior UATI. The target PCF receives the prior session information from the source PCF and retrieves the session information of the AT using the received prior session information.