The Middle Cambrian gypsum-salt rocks are widely distributed in the Bachu Uplift of the Tarim Basin. These rocks are good regional cap rocks. Fluid inclusion petrography, microthermometry and thermodynamic calculation were used to study the halite samples from well Bt5. According to the analysis of fluid inclusion petrography, the primary fluid inclusion assemblages were preserved during the alteration of tectonic movement and bury process. An average homogenization temperature of 29.9℃ was yielded, which is consistent with the paleo-temperature of surface water. According to the result of thermodynamic calculation, the increasing rate of internal pressure of the fluid inclusions in halite was low during the bury process and it was probably one of the important factors responsible for fluid inclusion preservation.