Abstract: Deep water surface casing cementing is a key part in deepwater drilling.The cementing wellhead sealed tube method is usually adopted but due to lake of theoretical research and design basis,the depth is not reasonable,which can easily lead to too much displacement or insufficient displacement,resulting in unqualified casing cementing quality,high plug in casing shoes stay,and solid casing string inside.In deep water surface casing cementing,a series of problems have been occurred,a set of suitable design theory and methods for cementing wellhead inside the sealed tube within the tubing string running have been put forward.With these methods,the different size of the string with different displacement efficiency of different water depth under different conditions can be calculated.Therefore,the corresponding reference charts can be drawn.The design method of the actual application effect is good,which can improve the efficiency of the deep water surface casing cementing operations.