The invention discloses the on-line measurement method of the parameters of a passive filter and a reactive power compensation device. The method comprises the following steps of firstly, on-line measuring the voltage and current of the passive filter to be detected and a reactive power compensation device branch in an on-line mode; carrying out a single-cycle Fourier analysis on the measured voltage and current and acquiring a fundamental voltage, a fundamental current, and each integer harmonic voltage and harmonic current; and according to the obtained fundamental voltage, the fundamental current, and the harmonic voltage and the harmonic current under a specific harmonic order, acquiring the inductance value and the capacitance value of the passive filter and the reactive power compensation device. By using the method, the operating parameters of the passive filter and the reactive power compensation device can be obtained simply and rapidly so that the states of the passive filterand the reactive power compensation device can be analyzed and the safe and reliable operation of the device can be ensured.