Provided in the embodiments of the disclosure are methods for transmitting data, a terminal device and a network device. One method includes: a terminal device receives semi-static configuration information sent by a network device, the semi-static configuration information being used for indicating an initial state of a duplicated-data transmission function of a Packet Data Convergence Protocol (PDCP) entity corresponding to a Radio Bearer (RB), and the initial state including an activated state or a deactivated state; and the terminal device establishes a first RB according to the semi-static configuration information. The methods, the terminal device and the network device provided by the embodiments of the disclosure help to improve the reliability of data transmission.