Abstract: The research on the change of irreducible water in high temperature and high-pressure gas reservoirs is usually based on the theory of oil-gas-water multiphase phase equilibrium. The research process involves the selection and calculation of complex phase model. The calculation process is complex, the calculation accuracy is not high, and the application is limited. In order to quickly and conveniently evaluate the change of irreducible water in the process of production, the experiment method of NMR on-line displacement detection is used to evaluate the change of irreducible water in the process of exploitation. Based on the experimental results, the variation chart of irreducible water saturation in the production process of high temperature and high-pressure gas reservoir in X gas field is established. The results show that the irreducible water saturation of high temperature and high-pressure gas reservoir in X gas field decreases obviously with the development of depletion exploitation, and the dynamic change of water-gas ratio in actual production is in good agreement with the experimental results, which further proves the reliability of the research results.