Abstract: For solving the problem of well leakage caused by the narrow mud density window drilling, at the deep water oil and gas field in Qiongdongnan Basin of the northern South China Sea, the calculation model has been established to analysis of wellbore stability in deep water conditions. In addition, we calculate formation pressure and collapse pressure and rupture pressure about L4 well based on the seismic data. It indicated that the collapse pressure increases with well depth and it is all less than formation pressure, so the formation pore pressure is as a lower limit of the safe mud density window. Further the rupture pressure increases with well depth and it has a minimum value just 1.02 g/cm3 at the bottom of the sea. The lower limit of formation pressure and rupture pressure just ranges from 0.021 g/cm3 to 0.092 g/cm3, the maximum value is 0.290 g/cm3. It indicates that the safe mud density window is narrow. According the actual situation of the target wells and ECD and the surge pressure, we recommended the range of drilling fluid density in different layers. The calculation results consistent with the actual drilling data and meet the actual needs. It indicated it is feasible about application the seismic data to calculate the safe mud density window, and it is of important significance for the area following drilling.