The Cascade H-bridge converter(CHB) is attractive for photovoltaic applications because of the advantages of its high modularity and high-voltage ability. There exists a large number of capacitors in CHB system, which determines the output waveform quality. Therefore, to get the health status of those capacitors is quite meaningful. In this paper, a capacitor condition monitoring method based on the recursive least square(RLS) method. The recursive equation of HB module capacitance value is proposed, which utilizes the feature that the ac side of the inverter only exchanges energy with the dc side capacitor in static var generator(SVG) mode at night. The online condition monitoring methods only need the capacitor voltage signals, which are already collected as control variables, and the overall cost is low. The effectiveness and accuracy of the proposed method are verified by simulation analysis.