Abstract: Through analyzing the characteristics of geology, difficulty in drilling engineering technique in the K Structure, and technical requirements for drilling fluid, the drilling fluid system of amphion polysulfonate/under-saturated salt water with density of 2.41 g/cm3 is developed according to the characteristics of downhole complicated accidents such as high mineralization of deep saline aquifer, high pressure factor, narrow safety density window, and easy pipe-sticking in this area. At the same time, the measures of reservoir protection are put forward for reservoir under deep seated salt, which are taken in the field and have significant effect. Three kinds of drilling fluid systems are used with the increase of depth in drilling of well 4 in this structure, including: amphion polymer system, amphion polysulfonate/under-saturated salt water system, polysulfonate/under-saturated salt water system with formation protection system. The refined control of rheological property of drilling fluid ensures the safe, rapid and accident-free drilling, final completion depth of 6,596 m and good show of gas and oil, which provides powerful technical guarantee for future development in the K Structure.