The invention discloses a circuit and a method for realizing automatic switching of a vehicle-mounted external audio channel. The voltage doubling rectifying characteristic of a voltage doubling rectifying circuit, the structural characteristic of a triode and the current characteristic under different working states are utilized, and automatic switching of a vehicle-mounted external audio channelis achieved in an I2C communication and A/D collection mode. The automatic switching circuit and the automatic switching method can be applied to vehicle-mounted multimedia terminal equipment with anexternal audio input function and are used for realizing automatic switching of external audio channels, so that a driver can listen to music of equipment such as a mobile phone at any time on the premise of safe driving; According to the invention, automatic detection and automatic switching of audio channels can be realized even in a relatively weak external audio signal, and the method has theadvantage of high detection precision.