In recent years, the green hydrogen production via water electrolysis has acquired significant attention, and the dynamic operation of electrolysis stacks has become a crucial research area. The electric double-layer capacitance (EDLC) is the primary factor indicating the performance of the stack and affecting the dynamic behavior of the electrolysis stack. High-frequency testing methods are commonly used to measure the EDLCs of electrolysis cells in laboratory, but these methods are not suitable for cost-efficiency and a bottleneck of the engineering power electronic devices. This study proposes a novel square-wave pulse current (SPC) method for determining the EDLCs of multi-cell electrolysis stacks using industrial rectifiers. The response of the stack subjected to SPC is analyzed based on a semi-empirical equivalent circuit (SEEC) model. The feasibility of the proposed method is verified through experiments on a 2 Nm/h alkaline water electrolysis stack, and an accuracy of 2.67% is obtained.