Abstract: Casing stress calculations in creep formation are the basis to take casing failure prevention measures. The current casing stress calculation method, which uses non-uniform formation stress as the boundary condition of cement sheath mechanical model, is inconsistent with the actual situation. The finite element mechanics model is put forward, which uses uniform formation stress as the boundary condition of casing stress calculation in creep formation. And the effect of cement sheath ellipticity and elastic modulus on the casing stress is analyzed. When the cement sheath elastic modulus is less than or equal to the formation elastic modulus, the maximum casing stress is in the same direction with the minimum horizontal in-situ stress, and on the contrary, it is in the same direction with the maximum horizontal in-situ stress. The peak casing stress increases as the cement sheath ellipticity increases, so the casing breaking may change from necking to collapsing. The cement sheath elastic modulus has a reasonable matching relationship with formation elastic modulus and borehole ellipticity, and it is the most effective way to reduce casing stress by changing the cement sheath elastic modulus in creep formation. The law would help deepen the understanding of casing failure mechanism and the implementation of prevention measures.