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; receiving a message 2 sent by the network device, where the message 2 includes uplink grant information, the uplink grant information includes first indication information, and the first indication information is used to indicate a largest transport block size TBS allowed for a message 3; 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; and sending the message 3 to the network device, where the message 3 includes uplink data transmitted based on the target TBS.