A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises starting a mobile reachable timer when a mobile device enters an idle mode, wherein the mobile reachable timer has a first timer value; accepting the emergency PDN connection; stopping the mobile reachable timer when the mobile device enters a connected mode to establish the emergency PDN connection; resetting the mobile reachable timer to a second timer value when a remaining time of the back-off timer is longer than a periodic area update timer; and restarting the mobile reachable timer according to the second timer value when the emergency PDN is disconnected.