Large non-anchored welded steel oil tanks are important equipments that are renowned for their extremely large structures, they are vulnerable to elephant’s foot buckling failure which can cause serious damages in seismic. Generally, locations of tank openings are at the first layer of shell course which are just close to elephant’s foot buckling position of oil tanks. While learning the effect of openings on elephant’s foot buckling of oil tanks, the welding residual deformation was assumed to be initial geometric imperfections. Numerical simulation method was adopted to analyze the effect of three factors (oil tanks with or without openings, opening sizes and opening positions) on elephant’s foot buckling critical load of oil tanks. Results show that the openings and reinforcement plates are beneficial to the increase of buckling critical load; the size and location of the openings have little effect on the buckling critical load.