Power capacitor bank is an important device, when it occurs a minor fault, if not be found in time, will gradually develop into a serious malfunction, causing damage to the capacitor bank. To solve this problem, this paper proposes a new method of early capacitor bank internal component failure warning: the LC oscillation frequency variation method based on early theoretical characteristics of a single component damage analysis. The method calculates the oscillation frequency by collecting discharge PT voltage and bus voltage, records the frequency of initial value, and then compares the variation in the process to propose early warning. The advantages of this method are of high detection precision, and can judge the fault phase and avoid the system voltage fluctuation. According to the warning method needs, an on-line monitoring and early warning device of capacitor bank is designed to realize the engineering applications of this method. A variety of data after the field operation also verify this warning method. Thus this device is convenient for the operation and the management of the capacitor banks, and provides a new way to find the early fault in time.