A seawater-based drilling fluid has been developed through selection of key additives such as fluid loss reducers， high-temperature stabilizers， and optimization of the components to deal with the problems encountered in deep drilling， such as instability of the borehole wall， unstable rheological properties of drilling fluid， and large fluid loss. The comprehensive performance of the drilling fluid was analyzed， and the results showed that the drilling fluid with density of 1.5g/cm³ had an apparent viscosity change rate of 2.5% after aging for 16h at 230℃， and the HPHT filtration loss was 23mL； indicating that the seawater drilling fluid could resist temperature up to 230℃ with good high-temperature stability， high-temperature rheological properties and low filtration loss at high-temperature and high-pressure， and it can meet the requirements of deep-sea high-temperature drilling.