Disclosed is a low-pass filtering system having a phase-lock loop comprising a Park transform circuit, a first low-pass filter, a second low-pass filter, an inverse Park transform circuit, a phase-locked loop filter, and a voltage-controlled oscillator. The Park transform circuit, the first low-pass filter, the second low-pass filter, and the inverse Park transform circuit form a phase detector of a phase-locked loop, since the low-pass filter system of the present invention has the phase-locked loop mechanism, the phase and amplitude of a output signal remain the same with those of the original AC input signal.