A cell activation/deactivation method in a wireless communication system and an apparatus therefor are disclosed. Specifically, a method for activating/deactivating a secondary cell (SCell) by a UE in a wireless communication system supporting carrier aggregation includes: receiving an SCell addition related message from an eNB; partially activating one or more first SCell when a partial activation related message for the first SCell from among a plurality of SCells added according to the SCell addition related message is received from the eNB; and fully activating one or more second SCell when a full activation related message for the second SCell from among the plurality of SCells added according to the SCell addition related message is received from the eNB, wherein normal SCell operations are applied to the second SCell and only some of the normal SCell operations are applied to the first SCell.