A communications system switching method and a terminal device establish, by the terminal device, a connection to a first communications system, run a first application, obtain a QoS value of a second communications system, determine 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. When the QoS value of the second communications system meets the QOS requirement of the first application, the terminal device disconnects from the first communications system and establishes a connection to the second communications system.