Provided are a method and device for performing data transmission capable of solving a problem in the prior art in which if a user equipment unit needs to perform transmission with a network, the user equipment unit needs to perform a cell switching process to enter another cell, and this results in higher signaling overheads for a terminal. In an embodiment of the invention, a new connected state is different from an idle state and a connected state. In the new connected state, a terminal can perform data transmission with a network side device. Moreover, after the terminal has moved to a coverage of another cell from a coverage of a cell currently accessed by it, the terminal can move to the new cell by means of cell reselection without performing a cell switching process.