Power capacitor device is a widely used reactive power compensation equipment in power transmission and distribution system which can easily have internal fault and therefore affects the safe operation of the power system. This paper studies the internal structure of the power capacitor and the possible fault of the internal elements, then establishes a mathematical model, and thus derives a criterion based on the percent change of voltage and current of the power capacitor, judging whether elements in the capacitor have fault or not, besides, this paper also puts forward an improved measure, and at the same time, verifies the criterion in Matlab Simulation.