An echo-cancelling device includes an audio input/output (I/O) terminal, an audio-receiving module, an analog-to-digital (A/D) converter, and a processor is provided. The A/D converter is electrically connected to the audio-receiving module. The processor is electrically connected to the A/D converter and the audio I/O terminal. The audio I/O terminal receives an external reference signal from an electronic device. The audio-receiving module receives an input audio and an output audio from the electronic device, and generates an analog input signal having the input audio and the output audio. The A/D converter generates a digital input signal according to the analog input signal. The processor cancels the output audio to generate a second (digital) sound signal according to the digital input signal and the external reference signal. Finally, the processor transmits the second (digital) sound signal to the electronic device via the audio I/O terminal.