Abstract: In cementing operation, it is not only needed to trip in casing to the designed depth, but also needed to trip in, trip out and rotate the casing to improve the cementing quality after cement injection. In some special cases, casing may be tripped out from the well. Casing string tension-torque analysis should be made to determine whether the strength of casing and the surface equipments meet the requirements. In this paper, the tension-torque model for casing in directional wells is introduced, and the strength verification method is developed. The casing string used in a directional well in Daqing Oilfield is analyzed. The result shows that the casing of the well has strength is high satisfactory strength. The operation of tripping in, tripping out and rotating casing strings can be implemented.