The current density of water reduction using a commercial Pt disk electrode was enhanced under 2.45 GHz microwave irradiation. The transitional alternation of temperature distribution was explained with a heat-transfer simulation using COMSOL Multiphysics based on the finite element method. Results show that most of the microwave energy was converted into thermal energy at the vicinity of the Pt electrode. A local high-temperature region enhancing the water-reduction reaction was hence formed.