The embodiments of the disclosure relate to a data transmission method, a terminal device and a Core Network (CN) device. The method includes that: a terminal device determines a target access type for uplink data in multiple different access types; the terminal device determines a target-type CN corresponding to the target access type according to a correspondence between an access type and a CN type; and the terminal device sends the uplink data by use of at least one data flow corresponding to the target-type CN. According to the data transmission method, terminal device and CN device of the embodiments of the disclosure, transmission efficiency may be improved.