The present application provides a data transmission method in a wireless communication system. A first security parameter indication is introduced into uplink data on a first bearer, and a base station can determine, based on the first security parameter indication in the uplink data, whether first security processing of the uplink data is performed using a first security parameter or a second security parameter, so as to select a corresponding security parameter to perform second security processing; and a second security parameter indication is introduced into downlink data, and UE can determine, based on the second security parameter indication in the downlink data, whether the first security processing of the data is performed using the first security parameter or the second security parameter, so as to select a corresponding security parameter to perform the second security processing.