This application provides a data transmission method, and an apparatus. The method includes: receiving a first downlink reference signal, and obtaining a first key based on the first downlink reference signal; sending a first uplink reference signal; and receiving a first message, determining a third key based on the first key and check information of a second key, and performing, based on the third key, security protection on data transmitted between a first communication apparatus and a second communication apparatus, where the second key is obtained by the second communication apparatus based on the received first uplink reference signal. According to the method, security protection can be performed on data transmitted in a random access procedure.