This application relates to the field of communication technologies, and discloses a transport block size determining method and an apparatus. The method includes: A terminal device determines, based on control information sent by a network device, K transmission occasions used to transmit a first data packet, where at least two of the K transmission occasions include different quantities of valid REs. Further, the terminal device may determine, based on a target quantity of valid REs, a TBS corresponding to the first data packet, where the target quantity of valid REs may be an average quantity of valid REs of the K transmission occasions, or may be a quantity of valid REs included in a target transmission occasion in the K transmission occasions.