Due to the limitation of the small and medium-modulus internal gear structure, it is difficult to use the traditional grinding process for the tooth profile processing after the tooth surface is hardened. Therefore, an EDM forming milling method was proposed. Due to the electrode rotation discharge, the electrode discharge area is large. The processing accuracy is less affected by electrode loss, and the products of electro-corrosion are better discharged. The processing device was designed according to the processing principle and method, and the basic process law was studied by single factor experiment. According to the results of the single factor experiment, a representative level was selected to design an orthogonal experiment, and the optimal machining conditions were studied through orthogonal experiments combined with surface roughness and machining efficiency. A comparative test is designed, and the experimental results show that compared with the traditional EDM processing method, the surface quality and processing efficiency of the workpiece are significantly improved.