Abstract: The problem of big data difference and the incapability of reading data caused by using inappropriate couplant existed during the process of measuring the thickness at the certain spot at high temperatures. Linear equation of sonic speed under high temperature in different materials was derived from linear regressive analysis on the experimental data of sonic speed both under certain temperature and the temperature below. The equation was applied to measure the thickness of a certain place on the tube under high temperature circumstances accurately and conveniently which could solve the problem of coincident large data effectively. Otherwise, through the research on the performance of different couplants, the appropriate couplants which were able to effectively couple under high temperature were screened.