Accidentally 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. Because of the occasional 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, my life and increases with the increase of coupling diameter. I compared with prefabricated thermocouple, although have many advantages, but life often below fabricated thermocouple. Because of thermocouple structure is simple, let I to be more careful when using, if use undeserved, influence of the spring, for my life and we also made some analysis, mainly the temperature of the environment and the quality of the protection tube, I choose good quality protection tube is very necessary.