A module system of a redox flow battery is disclosed. The system includes a first redox flow battery module, through which first and second electrolytic solutions circulate, a second redox flow battery module, through which first and second electrolytic solutions circulate, first and second storage tanks storing the first electrolytic solutions of the first and second redox flow battery modules, a first main pipe connecting the first redox flow battery module fluidically to the first storage tank, and a first transfer pipe and a first equilibrium pipe which are configured to allow for fluid communication between the first electrolytic solutions of the first and second redox flow battery module. The first main pipe has a diameter greater than that of the first transfer pipe.