The cooling medium choice is restricted due to no detailed regulations for temperature control as the specimen with gaseous cooling medium in the API, ASTM and the standards or specifications of China. The analysis and calculating are made to the physical process of the cooling. The preservation hold time are provided for DWTT specimen at different temperature in the gaseous cooling medium. It provide primary sustaining base to control the cooling process of gaseous cooling medium. The essential point of the precise means of temperature testing is brought forward at the same time.