Propose the grey relation theory for influence of electric milling on machining targets in achieving parameter optimization, when an engine cylinder head milling processing under plane milling tool. The experiment was designed via orthogonal experimental design technique. Some parameters such as machining distortion, surface roughness and machining efficiency were chosen as machining targets, and the milling speed, feed per tooth and milling depth were chosen as milling parameters, calculate and analyze the grey relational degree and relational coefficient between milling parameters and machining targets. The optimized milling parameters including milling speed of 380m/min, feed per tooth of 0.05mm, and milling depth of 1.5mm were obtained in single-object and multi-objects. Experimental results show that the machining distortion of 0.055mm, the surface roughness of 0.7964μm and the machining efficiency of 36.3 mm3/min are excellent according to the optimized designs, and the machining efficiency and the machining quality are improved greatly.