A communications device is capable of adjusting power consumed thereby and includes a receiver and a control circuit. The receiver includes an analog-to-digital converter and a decoding circuit. The analog-to-digital converter receives receive-data from another communications device, converts the receive-data into a digital signal, and provides the digital signal to the decoding circuit for decoding. Based on the digital signal, the control circuit obtains a signaling index that indicates a communications quality of the receiver. The control circuit determines an operating state of the analog-to-digital converter and the decoding circuit in accordance with the signaling index and a receiving index. When the signaling index conforms with a reference range, the control circuit reduces a reference signal so as to reduce linearity of the analog-to-digital converter in accordance with the receiving index, and reduces a decoding capability of the decoding circuit in accordance with the receiving index.