The present disclosure provides a method, system, and terminal device for data transmission in an unlicensed spectrum, effectively reduce mutual signal interference between different systems while meeting regulation constraints on use of the unlicensed spectrum. The method in the present disclosure includes: at a processing start moment of a terminal device in a current channel occupancy time window of a network device, when remaining duration of the current channel occupancy time window of the network device is greater than or equal to duration for the terminal device to transmit a to-be-sent data packet to the network device, selecting based on a user attribute of the terminal device and from a mapping relationship between a user attribute and a transmission mode; and sending the to-be-sent data packet to the network device in the selected transmission mode.