This application provides a method and an apparatus that are for transmitting data in a random access process. The method includes sending a message 1 to a network device. The method further includes receiving a message 2 sent by the network device, where the message 2 includes uplink grant information, where the uplink grant information includes first indication information, and where the first indication information is used to indicate a largest transport block size TBS allowed for a message 3. The method includes determining a target TBS in candidate TBSs based on the allowed largest TBS, where the target TBS is less than or equal to the allowed largest TBS. The method further includes sending the message 3 to the network device, where the message 3 includes uplink data transmitted based on the target TBS.