A circuit for reducing inductor magnetic-core loss is disclosed. The circuit includes a switch transistor, a PWM signal source connecting to the switch transistor, an inductor connecting to the switch transistor, a load connecting to the switch transistor, and a frequency adjustment circuit. The frequency adjustment circuit obtains PWM signals from the PWM signal source and modulates the PWM signals to be a new square-wave signals to be outputted to the switch transistor. The switch transistor is configured for setting the frequency of the square-wave signals as the operation frequency so as to control a duration for which the current flowing through the inductor. The operation frequency of the switch transistor may be adjusted by modulating the frequency of the PWM signals. As such, the duration for which the current flowing through the inductor may be controlled so as to decrease the current amount flowing through the inductor.