In order to study the effect of temperature on the bolt pretension of flange structure，the bolt pretension and sealing conditions of flange in liquified natural gas ( LNG) ultra-low temperature ball valve at different temperatures were analyzed by ANSYS finite element software，and the pretension states of bolt materials with different linear expansion coefficients at room temperature and low temperature were compared． The results show that when the bolt material is B8M and XM-19，the preload force increases with the decrease of temperature，and the pretension under the low temperature conditions increases by 24. 93% and 6. 28% respectively compared with normal temperature; when the bolt material is Inconel625，the pretension decreases with the decrease of temperature，and the pretension under the low temperature condition decreases by 26. 39% compared with normal temperature． At the same time，the variation of the sealing specific pressure of the metal spiral wound gasket with temperature exhibits the same trend with the bolt pretension．