Thermocouple as instrument of measuring temperature, can bend, high pressure resistance, heat fast response time and strong and durable and many other advantages, and usually display meter, recording meter and electronic regulator, direct measurement of various production process from 0 ℃ ~ 800 ℃ in the scope of liquid, steam and gas medium, as well as the surface temperature of the solid. Due to the thermocouple with casing protection and isolated from the external environment, so the casing material had a great influence on the life of the thermocouple, thermocouple wire and metal casing must choose according to use. When the material is selected, and the life of a thermocouple with increased with increasing the diameter of the thermocouple. Thermocouple compared with prefabricated thermocouple, although have many advantages, but life often below fabricated thermocouple. Is because of thermocouple structure is simple, make thermocouple to be more careful when using, if use undeserved, influence of the spring, for the service life of the thermocouple we also do some analysis, mainly to see the quality of the environment and protecting tube temperature, choose good quality protection tube is very necessary.