A communications system switching method and a terminal device are provided. The method includes: establishing, by a terminal device, a connection to a first communications system, and running, by the terminal device, a first application; obtaining, by the terminal device, a QoS value of a second communications system; determining, by the terminal device based on a preset policy and a QoS requirement of the first application, whether the QoS value of the second communications system meets the QoS requirement of the first application; and if the QoS value of the second communications system meets the QoS requirement of the first application, disconnecting, by the terminal device, from the first communications system, and establishing a connection to the second communications system. In this manner, the terminal device can select a proper communications system based on a currently running application. This is relatively flexible and intelligent.