A signal transmission method, a terminal device and a network device are provided. The method includes: a terminal device receives downlink control information, (DCI), the DCI carries resource reservation indication information, and the resource reservation indication information indicates the terminal device whether reserve a physical resource is to be reserved in a time-domain resource unit; the terminal device determines a first physical resource used for transmitting a first signal in the time-domain resource unit according to the resource reservation indication information, or determines a second physical resource used for an uplink and downlink guard period in the time-domain resource unit according to the resource reservation indication information.