In this work, the viscosity of an organic electrolyte solution is quantified and its effect is investigated on the performance of an aqueous organic redox flow battery (AORFB). For a viologen derivative, the solution viscosity is found to be highly dependent on both the state of charge (SOC) and the concentration, which can be attributed to the strong intermolecular interaction in solution. The correlation between the viscosity and the SOC is then developed for different concentrations of the electrolytes. A 2D model with the effect of variable viscosity is developed to elucidate the mass transport and electrochemical reaction processes in the flow battery. It is found that the variable viscosity yields non-uniform distribution of the local velocity and concentration in the porous electrode and lowers the performance of the flow battery. Based on the above findings, the electrode porosity is optimized and a gradient porosity is developed to match the mass transport with the electrochemical reaction in flow batteries. This work not only elucidates the key role of viscosity in the flow battery for the first time, but also provides a rational design of the electrode structure to balance the mass transfer and electrochemical reactions.