Embodiments of the present application provide a transmission data method, a terminal device, and a network device, capable of reducing the complexity of blind detection of a Physical Downlink Control Channel (PDCCH) by a terminal device. The method comprises: a terminal device receives a Slot Format Indicator (SFI) sent by a network device; the terminal device determines, according to the SFI, at least one of a slot structure in a time cell and the length of time applicable to the slot structure; and the terminal device detects a PDCCH according to the SFI.