Mode switching is essential in a unitized regenerative fuel cell system. The switching of electricity, reactants, and reversible electrochemical reactions occurs, thereby affecting cell performance. During model switching, insufficient reactant supply may cause severe concentration, especially along the width of the catalyst layer which is not currently considered in a low-dimensional model. To fill this gap, a three-dimensional simulation in a proton exchange membrane unitized regenerative fuel cell is performed and validated to investigate the dynamic responses of current density and mass transfer. Results certify that reductions on current density and hydrogen generation are severe, as the cell is switched to a high electrolysis voltage. The analysis of species transfer along the width of the porous layers indicates that the deterioration is attributed to the occurrence of serious concentration polarization which is caused by the severe local water starvation on the catalyst layer, especially below the rib. A high inlet velocity for reactant and low fuel cell voltage are conducive for remitting local water starvation to improve cell performance effectively. However, a slight deterioration cannot be avoided completely before the water supply arrives, which is limited by the high rates of electrochemical reactions at a high electrolysis voltage.