Embodiments of this application provide a data transmission method, a terminal device, and a network side device. The method may include receiving, by a terminal device, a first identifier (ID). The method may also include indicating, by the terminal device, mobility information of the terminal device to the first network side device. Furthermore, the method may include receiving, by the terminal device, target grant free information indicated by the first network side device, where the target grant free resource is a dedicated grant free resource or a universal grant free resource, data information sent by the terminal device to the first network side device using the dedicated grant free resource includes data sent by the terminal device, and data information sent by the terminal device to the first network side device using the universal grant free resource includes data sent by the terminal device and the first ID.