Abstract: The treatment of lost circulation in deep water drilling is difficult, long cycle and high economic cost. Improper treatment can easily lead to accidents such as sticking, wellbore collapse and blowout. In order to improve the treatment efficiency of lost circulation in deep water drilling, this paper analyzes the root causes of lost circulation during deep water drilling in South China Sea, and proposes a new method of treating lost circulation with composite plugging cement slurry. The plugging mechanism of the composite plugging cement slurry is analyzed, and the reliability of the method is verified. Finally, the engineering operation experience of dealing with lost circulation in reef limestone is formed, and the technical measures to prevent further deterioration of lost circulation events into stuck drilling, well collapse and blowout accidents are put forward, which provides reference and enlightenment for subsequent treatment and prevention of lost circulation in deep water.