The present invention relates to a signal duty cycle adaptive-adjustment circuit and method for a receiving terminal. In one embodiment, the circuit includes an analog level comparison circuit, a preprocessing circuit, a first path switch, a second path switch, a decoding circuit, a parameter extraction and estimation circuit, an error generation circuit, a filter feedback circuit and a digital-to-analog conversion circuit. The analog level comparison circuit receives a valid signal according to a reference level to generate a duty cycle signal. The preprocessing circuit preprocesses the duty cycle signal. When the first path switch is turned on, the parameter extraction and estimation circuit acquires duty cycle information from the duty cycle signal to generate a duty cycle deviation. The error generation circuit processes the duty cycle deviation to generate an error signal. The filter feedback circuit and the digital-to-analog conversion circuit filter the error signal and then convert the error signal into an analog voltage signal, which is connected to the analog level comparison circuit to serve as a reference level. When the second path switch is turned on, the decoding circuit decodes the duty cycle signal.