The ultrahigh temperature drilling trajectory measurement instrument is a new type of storage inclinometer system specially developed for drilling trajectory measurement in high temperature environment，with the vacuum flask as the main means of insulation from high temperature environment, since the vacuum flask has a large length/diameter ratio with the internal effective use length up to 2.0m. In order to understand the law of temperature rise in the axial direction of the vacuum flask when the external environment temperature is fixed, and provide a basis for the reasonable arrangement of the probe tube structure, this experimental study has been conducted. Through the laboratory test of two vacuum flasks, the internal temperature rise value of the vacuum flasks in different ambient temperatures was obtained. Through the analysis of the measured data, the internal temperature rise law of the vacuum flasks was found to include three main aspects: first, the temperature rise value is basically linear with time, and the temperature rise rate increases with the ambient temperature; second, the temperature rise value is a function of the ambient temperature in a cubic curve; third, the internal temperature rise value of the vacuum flask gradually decreases from both ends toward the middle, and the temperature rise value of the middle part is significantly lower than that of the two ends. Based on the test results, the layout of the probe tube structure has been rationalized with specific measures taken to ensure that the working temperature index of the instrument meets the specified requirements. The test results can also provide a reference for the structural layout of other similar hightemperature instruments.