Nitrogen drilling is one of the effective ways to develop tight sandstone gas reservoir because of its unique advantages. The risk of formation water production in drilling will limit the large-scale application of nitrogen drilling technology, so how to accurately predict or monitor formation water production has become a "choke" technology in the development of nitrogen drilling. Comprehensively considering gas production and water production, coupled with wellbore pressure field and temperature field, the model of relative humidity distribution in the whole wellbore was established and numerically solved, it realized the fast quantitative evaluation of key parameters such as downhole pressure, temperature and humidity. During nitrogen drilling in DY105 well of Xujiahe in western Sichuan, the water-out condition of formation was judged to guide on-site construction. The established relative humidity distribution mode is of great significance to ensure the safe operation of nitrogen drilling in tight sandstone water-bearing gas reservoir.