As high temperature and high pressure environment both have a very important influence on production logging instrument performance, especially the temperature. The moisture content meter is the core of the coaxial phase method for water detection instrument. The research of the heatresistant circuit has a very important significance for the whole instrument. The phase discriminator is an important part , and it is composed of LM139 voltage comparator and D trigger,they are designed and optimized based on the principal of the moisture content. The circuit is realized by PCB method.Through the temperature experiment has reached the requirement of 140℃, and it improved the instrumet temperatureresistant level.