Vanadium flow battery is emerging as one of the most promising candidates for large scale energy storage application. The major restriction on the route to commercialization is the high cost of the system. One of the effective ways to lower the cost of a vanadium flow battery system is improving the power density by reducing the battery polarization, which can be realized by controlling the electrode compression ratio reasonably. An overall polarization model, in consideration of the ohmic loss, activation loss and mass transport loss, was proposed for the first time. Then, the polarization model was coupled with a previously developed three-dimensional model to obtain a mechanistic understanding of the relationship between the electrode compression ratio and battery polarization. By combining numerical simulation with experiments, the effect of the compression ratio on the polarization and subsequently the battery performance was explored systemically and deeply, and the performance superiority of the battery assembly with the obtained optimal compression ratio was verified by experiments.