During well testing and production, the helical buckling often occurs near the lower end of tubing and friction appears. When loading continues or temperature increases, the tubing deformation will develop under post buckling condition, and it is difficult to describe the axial force with analytic method, that is why numerical method is needed. In this paper, plastic incremental theory in plastic mechanics is used to calculate the axial force distribution of buckled tubing strings, according to which the final axial force and deformation are determined by both the initial state and the loading process. It is found that the axial force distribution of tubing with friction is changed significantly and is strongly dependent upon the operation steps. The friction can prevent the large loads caused by high pressure and/or high temperature from spreading so that only a small part of tubing deforms severely. This calculation method was verified by experiments and has been used in oil fields.