Most of the damage of oil well casing is caused by non-uniform load, the non-uniform of external load and some other factors greatly reduce the bearing capacity of the case. There is important practical significance to reveal the influence of these factors on the collapse resistance of casing. According to the characteristics of hoop stress on casing under non-uniform elliptic load condition, a calculation model of hoop stress for ideal casing is established by the theory of material mechanics. Analysis is made on the regularity of stress distribution, the results show that the dangerous point is on the inner wall of casing by 90 angle between circumferential direction and the maximum horizontal stress, which has good consistency with the finite element numerical simulation result. Based on the stress state at the dangerous point, the new formula is proposed to calculate the equivalent external pressure on casing in different non-uniform loading. The calculation shows that non-uniform of elliptic load has great influence on casing equivalent external pressure; the more the non-uniform and the larger diameter-thickness ratio are, the poorer the bearing capacity of external load is and more easily collapse failure of casing happen.