A voltage regulating circuit including an error signal generator, a comparator, a switch unit, a low-pass filter and a modulating unit is provided. The error signal generator respectively receives a reference voltage signal and a feedback signal, and generates an error signal. The comparator respectively receives the error signal and a comparing signal, and generates a pulse width modulation signal. The switch unit regulates an input voltage signal to generate an output voltage signal according the pulse width modulation signal. The low-pass filter filters out the high frequency of the output signal and produces the feedback signal. The modulating unit is coupled to the low-pass filter and the error signal generator for regulating a transient voltage of the output voltage signal.