Drilling technology of ultra-deep well is still the focal point of drilling research, there are many difficulties in ultra-deep well drilling, such as bottom hole with high temperature and high pressure, complex geology, borehole with sharp dogleg, etc. These technical difficulties put forward higher request in casing running, well quality must be controlled strictly in order to run larger diameter combination casing string successfully. Well quality is closely linked with the drifting drilling assembly design. The accurate stiffness ratio calculation is the key prerequisite of drifting drilling assembly design. In this paper, a new mechanical model is applied to simulate deflection of the drifting drilling assembly. The solution of variable cross-section beam’s equivalent stiffness is applied to solve the stiffness of the drifting drilling assembly. Modified stiffness ratio calculation method for drifting drilling assembly and casing string is proposed. In the process of calculation formula derivation, the conservation of energy law has been used multiple times. At last, the VB computer program is compiled, which contains the modified calculation method. The computer program calculation value is larger than the conventional method’s calculation value. Modified calculation method is more reasonable, which has important significance for drifting drilling assembly optimal design.