Abstract: A mechanical model of casing cutting in deepwater is proposed based on the model of long pole milling. Through this model, the cutting torque of drill string and the maximum permitted pump stroke can be obtained. It is concluded that the relationship between the proportion of the cutting torque in the drive torque and the depth of water during operating provides some guidance for forming the judge index to determine whether the casing pipe is cut off completely in the deep water. Combined with the field data of two wells in the South China Sea, the model is verified. The results can be used for providing a theoretical reference for selecting the appropriate pump stroke and a technical guarantee for deepwater abandonment operations to improve the cutting speed and the time eficiency.