A simple dynamic model, taking into account the change of proton concentration and dynamic diffusion of vanadium ions, is developed to predict the performances of the vanadium flow battery (VFB). The model is verified via the cycle experiment on the single cell battery assembled with Nafion 115 membrane. The simulated voltage of the VFB single cell during a charge-discharge cycle is in agreement with the experimental results. The efficiency and capacity vs. number of cycles evaluated by this model agree quite well with the experimental data as well. Comparing with the model neglecting the proton concentration change and dynamic diffusion of vanadium ions, the presented model can better forecast the performances of the VFB.