Abstract: To improve the utilization rate of plasma active species, in this study, a closed non-uniform air gap is formed by a flowing water film electrode and a sawtooth insulating dielectric layer to realize the diffuse glow discharge in the atmosphere. Firstly, the electric field distribution characteristics of non-uniform air gap in the sawtooth dielectric layer are studied, and the influence of aspect ratio on the characteristics of diffuse discharge plasma is discussed. Subsequently, the effects of wire mesh, the inclination angle of the dielectric plate, and liquid inlet velocity on the flow characteristics of the water film electrode are analyzed. The results show that the non-uniform electric field distribution formed in the sawtooth groove can effectively inhibit the filamentous discharge, and the 1 mm flowing water film is directly used as the electrode, and high-active plasma is formed directly on the lower surface of the water film. In addition, a plasma flowing water treatment device is built to treat the methyl orange solution and observe its decolorization effect. The experimental results show that after 50 min of treatment, the decolorization rate of the methyl orange solution reaches 96%, which provides a new idea for industrial applications of wastewater treatment.