A data transmission method, a device, and a data transmission system are provided to implement local interaction between two terminals when the two terminals are served by different UPF entities. The method performed by a first user plane function entity includes receiving a data packet from a first terminal through an uplink path corresponding to the first terminal, where the data packet carries addressing information of a second terminal; determining path information of a second user plane function entity based on information about the uplink path corresponding to the first terminal and the addressing information of the second terminal; sending the data packet to the second user plane function entity based on the path information of the second user plane function entity; and sending the data packet to the second terminal through a downlink path corresponding to the second terminal.