The present invention discloses a method and device for controlling uplink transmit power of a terminal in a wireless access system that supports carrier aggregation/multiple cells. In particular, the method may include receiving a first TA value and a second TA value for a first timing advance group (TAG) including one or more component carriers and a second TAG including one or more component carriers, adjusting uplink transmission timing of the first TAG and the second TAG by using the first TA value and the second TA value, resetting a transient period between a first group of component carriers and a second group of component carriers exceeds a power control requirement, and controlling uplink transmit power according to the reset transient period.