Embodiments of this application provide a data transmission method, a terminal device and a network device, to reduce complexity of blindly detecting a PDCCH by the terminal device. The method includes: receiving by a terminal device, an SFI sent by a network device; determining, by the terminal device according to the SFI, at least one of a slot format in one time unit and a time length suitable for the slot format; and detecting, by the terminal device, a PDCCH according to the slot format corresponding to the one of the one or more slot format indices indicated by the SFI.