Abstract: A site investigation has been made on the rupture accident of a 10 000 m³ vertical vault oil tank (the roof ruptured during flusing with vapour). The investigative and analytical results show that there are three main causes for the rupture accident: ① the tank suffered from a raging fire before commissioning, which made the material performance change and made bending rigidity reduce; ② the tank's windward side ruptured in the rainstorm; ③ a subatmospheric pressure formed when the tank's temperature suddenly fell in the rainstorm during heating the tank with vapoure, and the subatmospheric pressure exceeded the limitation. Three measures are put forward in order to protect against the accident, and it is considered that it is necessary to implement and improve the industry standards concerned in tank cleaning.