Abstract: Objective With the increasing demand for petroleum and gas and the development for the associated technology, the record is constantly broken in the drilling depth. The in-situ stress increases with the drilling depth, leading to severe casing failure issues in the ultra-deep wells. In order to improve the bearing capacity of casing under high in-situ stress, large-wall thick high-strength casing is traditionally employed but without evident improvement. Methods In this paper, the finite element model of casing-cement sheath-formation system is used to prove the presence of arching effect in the casing system and the protective effect of flexible cement sheath on the casing. On this basis, a series of parameter analyses are conducted to study the influence of the elastic moduli of strata and cement sheath and the thickness of cement sheath on the stress transferring mechanism in the system. Results and Conclusions The results show that the soft cement can make the formation bear the in-situ stress by allowing the formation near the casing to deform with limited movement, while hard cement resists in-situ stresses by limiting the formation deformation near the casing. In the former scenario with active arching effect, the smaller elastic modulus of the cement sheath leads to the smaller stress in the casing. In the latter scenario with passive arching effect, the larger elastic modulus of cement leads to the smaller stress in the casing. It is worth noting that the stress reduction in the casing in the scenario of active arching is more obvious than that in the scenario of passive arching. Apart from the effects of the elastic modulus of the cement, the Poisson’s ratio and thickness of the cement also has a limited influence on the casing stress, but the extent of their influence is not as significant as that of the elastic modulus of the cement sheath. In view of this, it is suggested that the selection criterion for the cement of in the ultra-deep well cementing should be whether the formation has shear strength to form arching effect rather than the stiffness of the formation.