Deep-well combined drill string technology has become one of the most important scheme to solve the super-long drill string used for ultra-deep drilling project, which is related to equipment capacity, material property, drilling conditions and string mechanics. In this paper, the application and development of combined drill string technology in geological drilling are summarized systematically, the design scheme and its limitation length are discussed in detail on the basis of strength theory. The results show that the drilling mission with the depth of 13000m could not be finished by simply using pure steel combined drill string with the final borehole diameter being 216mm. By using drill string consisting of V150 steel drill pipe & titanium alloy drill pipe and S135 steel drill pipe & aluminum alloy drill pipe, the limitation depth and the weight of each combined drill string are respectively 13484m, 18783m and 360.5, 324.3t; in this way both of them own application reliability and advantage. These results have reference significance to the design and selection of deep-well combined drill string used for ultra-deep drilling project.