Abstract: Under non uniform in-situ stress casing collapse failure often happens. Especially in soft rock the problem is more serious. So the mechanical model of the continuous body composed of soft formation, sheath and casing considers the contacting situation of the boundary between the casing and the sheath, and the stress and displace equations are derived by the semi-analytical method. The expression of the stress and displacement of casing and cement sheath were obtained. Taking the soft rock for an example, the effect of the contact state of cement sheath boundary, formation mechanical parameters, geometrical parameters of cement and casing geometrical data on casing’s force were studied. The results show that casing’s force is desirable under complete contact, and casing’s force is the worst under smooth contact. So enhancing the mechanical property of cement sheath and increasing the wall-thickness of casing can improve casing’s force.The model and mechanical analysis program of this paper provide reference for the engineering design of well cementing.