A data transmission method includes: receiving, by a network device, uplink data sent by a terminal device based on a grant-free manner; and sending, by the network device, a response message for the uplink data to the terminal device according to a type of the response message for the uplink data, where the type of the response message for the uplink data is a first response type or a second response type, the first response type is that each time the terminal device sends the uplink data, the network device sends the response message for the uplink data to the terminal device once, and the second response type is that after the terminal device sends the uplink data for a plurality of times, the network device sends, to the terminal device once, the response message for the uplink data that is sent for the plurality of times.