As the downhole has complex and changeful environment, high temperature and high pressure environment has a very important influence on various kinds of instrument performance. The research of the coaxial phase method for water detection instrument high temperature circuit to improve the instrument performance has very important significance. According to the measuring principle ,requirement of the environment and the whole project, designed and optimized the frequency mix circuit. Through the temperature experiment has reached the requirement of heatresistant at 140℃ and reduced the power consumption , eliminated the influence of temperature variation on the mixer measuring phase.