Abstract: Based on the characteristics of roof-to-shell connecting structure of vertical fixing roof oil tank, this paper calculates the instable pressure of weak connection between roof and shell in accordance with API 650, Appendix F-6. It also discusses the critical condition in which tank bottem would not be lifted, and explains its own position on the question of whether urgent gas relieve valve shall be installed if roof and shell of oil tank utilize weak connection and unweak connection.