A hardware-in-the-loop (HIL) simulation device is provided, which includes a processing circuit and a pulse-width modulation (PWM) signal observation circuit. The PWM signal observation circuit includes an energy storage unit and the energy storage unit is coupled to the processing circuit. A signal source transmits a PWM signal to the processing circuit and the PWM signal observation circuit, and the energy storage unit is charged when the PWM signal is at high level. The processing circuit detects the voltage of the energy storage unit when detecting the falling edge of the PWM signal so as to calculate the duty cycle of the PWM signal.