A data transmission method and an apparatus are provided. In this solution, one contention resource group may be divided into a plurality of terminal device groups, and terminal devices with a same DMRS may exist in two different terminal device groups. Even when terminal devices with a same DMRS exist in two different terminal device groups, interference can still be avoided. Therefore, according to embodiments of the present invention, a quantity of connected terminal devices in one contention resource group is increased while good communication quality is obtained.