Abstract: In this paper, the leakage accident of a 1 000 m³ diesel tank in an oil tankfarm is discussed, based on the mathematical model of soil mechanics and assuming that the settlement deformation of tank bottom is a linear deformation half-space under uniformly distributed load when it is filled. The causes and mechanism of this accident are concluded: due to the settlement difference between the tank wall and the tank bottom center, the siphons with both ends rigidly fixed at aforesaid positions are subject to strong stress which may concentrate in the location where the tank body deforms abruptly. When the stress value is greater than the yield limit and tensile strength of the material, the structure of tank will deform plastically and then break, leading to leakage accident. Finally, some improvements are proposed, such as modifying the design of sump and siphon, improving the quality of sump prefabrication and installation and strengthening the management of tank operation.