The invention relates to a method for utilizing a relay protection device to realize high voltage parallel capacitor online monitoring, and belongs to the field of electric power system high voltage parallel capacitor operation monitoring and relay protection. According to the technical scheme, firstly original functions of the relay protection device are utilized to measure the current and voltage of a high voltage parallel capacitor in real time; secondly, the relay protection device performs an original relay protection function; thirdly, the capacitance of the high voltage parallel capacitor is calculated by the measured current and voltage; and fourthly, the calculated capacitance is compared with a preset setting value, and the health state of the high voltage parallel capacitor is judged. According to the invention, no external auxiliary devices are added, the high voltage parallel capacitor online monitoring function and the relay protection device share a software and hardware platform, the capacitance of the high voltage parallel capacitor is calculated in real time, the health state of the high voltage parallel capacitor is evaluated in real time, and the online monitoring level of the high voltage parallel capacitor is effectively improved.