Abstract: Formation creep will make the external pressure on the casing get up to the overburden pressure in the process of well production. When the external extrusion load exceeds the casing compressive strength, the casing will be yielded and deformed or even cracked. The prevalent interaction model between creep formation and casing has some theoretical mistakes and can not be used for calculating the external pressure of casing. Based on the well construction procedure and mechanics principle, a correct interaction model between creep formation and casing was established. If the formation is rigid or elastic, earth stresses can not exert on the casing. If the formation is of creep property, the external pressure on the casing will be the overburden pressure after a long time. The non-axisymmetry deformation of casing detected from well site is mainly resulted from non-uniform thickness,non-uniform material of casing, scraping damage on casing and clay swelling in one side of casing, but not from the non-uniform earth stresses.