Abstract: In order to solve the problems occurr while drilling ultra deepwater,a new subsea mudlift drilling(SMD) technology is proposed by the oil industry.As a new technology used for deepwater and ultra deepwater drilling,the SMD technology demands for update hydraulic calculation methods and theories.Smaller reflux pipelines are used to transport drilling fluid back from the seabed,while risers are full of seawater.As a result,two pressure gradients develop in the backwatering circuit loop.According to the service conditions and characteristics of the SMD technology,a hydraulic calculation formula is deduced,as well as a hydraulic calculation program is established.The characteristics of the SMD system are analyzed based on the calculation examples,laying a theoretical basis for deepwater drilling operation.