A wireless communication system is disclosed. More particularly, a control method and device based on multiple priorities in a wireless communication system is disclosed. The method of controlling a back-off timer in a terminal configured with multiple priorities, which includes transmitting a first non-access stratum (NAS) request message set to a first priority level, starting the back-off timer configured by a network, and transmitting a second NAS request message, which is not set to the first priority level, while the back-off timer runs.