A method, an apparatus and a system for selecting a downlink primary carrier for transmitting data are disclosed. The method includes the following steps: A terminal obtains primary carrier selection information configured by a network; and the terminal determines a downlink carrier corresponding to an uplink carrier of the terminal as a downlink primary carrier for transmitting data according to the primary carrier selection information. In the embodiments of the present invention, the network selects a downlink carrier corresponding to an uplink carrier of the terminal as a downlink primary carrier for transmitting data. The terminal needs to detect only the downlink primary carrier, and the downlink carrier needs to be switched or detected only if the primary carrier receives a signaling message for detecting or receiving other carriers, so it is not necessary to switch or detect the downlink carrier frequently, which reduces battery consumption of the terminal.