This paper briefly introduces the application background and working principle of the double wheel milling trench cutter. In view of the probleum happened in the process of underground diaphragm wall trench in hard rocks using double wheel milling， this paper analyzes the misunderstanding of the mere milling method at the present stage， and the disadvantages of the pure milling method in the hard rock working condition， concludes that mere milling in hard rock can not give full play to the core advantages of the double wheel milling cutter， and then puts forward a drilling and milling combination method applied to trenching in hard rock. The advantages of the combined construction method are analyzed， and based on the principle of parallel operation， the key step of the construction method “drilling”is emphatically introduced from the selection of the guide hole construction method， the guide hole arrangement and the hole diameter selection. Three kinds of drilling arrangement methods suitable for different diameters， different rock strengths and different drilling equipment are proposed. In regard to the relevant cases， the practical application of the construction method and the benefits achieved are verified and explained. In the end， it expounds the purposes of “drilling” and “milling”， and defines the primary and secondary relationship between them.