The Cockcroft-Walton (CW) circuit is used in high-voltage low-current apparatuses, such as electron beam irradiation devices and insulation testing devices. The parasitic capacitance of the CW circuit is not considered in the design of a high voltage resonant converter. In the past, it has been clarified that the equivalent capacitor of the CW circuit can be utilized as a resonant capacitor to boost the output voltage. This study derives a theoretical quality factor and an output voltage of a resonant CW circuit, considering the equivalent capacitance and the equivalent conductance. Furthermore, it is revealed that parallel capacitors can improve the quality factor and the output voltage of the CW circuit. Experimental results obtained from the 5-stage resonant CW circuit verify the validity of the theoretical analysis.