With the implementation of deep oil and gas exploration， high temperature hot dry rock exploration and deep earth exploration strategy， the drilling fluids have been in high temperature and high-pressure environment for a long time， and the stability of water-based drilling fluids under ultra-high temperature and high pressure is a key technology for ultra-high temperature well drilling. In this paper， the technical problems faced by drilling fluids in ultra-high temperature scientific drilling engineering are analyzed， and the design approach for high density water-based drilling fluids formula resistant to 240℃ ultra-high temperature are proposed. The key additives for 240℃ ultra-high temperature and high density water-based drilling fluids were selected by single factor method. Using the synergistic effect of different high temperature additives， a set of ultra-high temperature and high density water-based drilling fluid formula with temperature resistance of 240℃ and density of 2.0g/cm³ was preliminarily developed. The results show that after aging at 240℃ for 16h， the optimal formulation of drilling fluids have good rheology and filtration reduction performance. Its apparent viscosity change rate is less than 30%， and the filtration loss at 180℃ is less than 24mL.