A direct current bus capacitor monitoring method based on a variable discharge network in the invention comprises the steps of carrying out voltage sampling on direct current bus capacitor voltages to obtain a voltage decrease curve of a direct current bus capacitor and carrying out on-line monitoring on the direct current bus capacitor according to the voltage decrease curve. According to the invention, sampling is carried out on the voltage of the direct current bus capacitor to obtain the voltage decrease curve of the three-phase frequency converter direct current bus capacitor so as to further accurately calculate the capacitance value of the capacitor and an equivalent series resistance. The method of the invention is a non-intruding type direct current bus capacitor on-line monitoring scheme; the realization is simple; the cost is low; no current sensor is needed; the capacitor does not need to be dismounted; the method is especially suitable for being used in power circuits such as an industrial frequency converter, is suitable for monitoring aging states of electrolytic capacitors, and is simultaneously suitable monitoring aging stats of thin film capacitors to realize on-line monitoring of health statuses of the thin film capacitors.