Generally, photon of Microwave (MW) electromagnetic waves have long been thought to be lower energy, which could not excite metals or semiconductor materials to generate electric current and electron-holes pairs (e-cb + h+vb). In this paper, we report an unexpected, Microwave "photoelectric effect", when MW irradiates on the semiconductor materials, leading to generate electric current and electron-holes pairs (e-cb + h+vb), on the semiconductor materials and on the MW catalyst. Further, we show that the action mechanism of Microwave "photoelectric effect" made water adsorbing on the surface of Microwave catalyst transform into hydroxyl radical (∙OH). Thus, this study has revealed the principle of generation Microwave "photoelectric effect" under MW irradiation, and the mechanism of MW catalytic oxidation degradation of organic in the wastewater and the mechanism of MW reduction method for preparation of nano-particle metal supported catalysts. Our findings challenge the classic view of MW irradiation only as heating method, which cannot excite to produce electric current and electron-holes pairs (e-cb + h+vb). Our findings will open new field to use MW technology for MW catalytic oxidation degradation of organics in the wastewater, and for MW reduction method of metal supported catalysts preparation.