The present disclosure provides a signal transmission circuit, a signal transmission system, a signal transmission method, and an intelligent terminal. The signal transmission circuit includes an earphone interface circuit, a comparison circuit, and a control circuit. The earphone interface circuit is configured for connection to an audio receiving device. The comparison circuit is configured to acquire a voltage value corresponding to an impedance of the audio receiving device via the earphone interface circuit, the voltage value is compared with a reference voltage of the comparison circuit, to output a comparison result to the control circuit. The control circuit judges whether the audio receiving device is a digital audio device according to the comparison result, and switches the earphone interface circuit to digital output when the audio receiving device is determined to be a digital audio device.