A data transmission method and a device are disclosed and relate to the field of communications technologies, so that the technical solutions can reduce power consumption of the second device during the channel detection. The method includes: determining, by a first device, a candidate-moment set; determining, by the first device, a target candidate moment from the candidate-moment set; and sending, by the first device, data on a channel according to the target candidate moment. The method and the device are used for data transmission.