The invention provides an assembling parallel capacitor fault online monitoring diagnosis method. The method comprises the following steps that a voltage and current measurement unit in a data acquisition system collects a phase voltage and a phase current of an assembling capacitor and a differential current signal of bridge circuit protection; a temperature measurement unit detects a temperature of a hottest spot in the assembling capacitor; a data acquisition module transmits an on-site signal to a background upper computer through an optical fiber; and the upper computer receives a signal through a Modbus protocol, a background analysis software system adopts a longitudinal and transverse comparison combination fault determination method to carry out analysis and determination on the signal. In the technical scheme provided in the invention, an operation state of the assembling capacitor is monitored in real time, whether the capacitor has a fault is analyzed and assessed, and early stage detection and early warning to assembling parallel capacitor medium degradation are realized.