With the occurrence of reversible electrochemical reactions, mode switching considerably affects the electric performance of unitized regenerative fuel cells (URFCs) owing to the complicated mass and heat transfer. Although limited researches have been done, no such studies on mass and heat transfer through a three-dimensional view are envisioned during mode switching. A three-dimensional full-cell model was developed and validated to study the dynamic characteristics of a proton exchange membrane-based URFC during mode switching. Mode switching was performed by changing operation voltage from 0.60 to 1.65 V. Results showed that species and heat transfer affect the electric performance of the cell during mode switching, especially through the third dimensional. Local water starvation occurs on oxygen side catalyst layer and thus results in slight reduction on current density and hydrogen generation. Restricted to heat transfer capacity through ribs, heat transfer process adds total response time in URFCs. Heat flux and surface heat transfer coefficient are forecasted on the hydrogen and oxygen sides. A total time of 4 seconds is essential for URFC reaching a new relative balanced state.