Abstract: The purpose of this paper is to analyse the additional axial force and its relative value caused by supposing the neutral axis through the sectional centroid of a cross section of a beam. Based on the balance condition of the cross section, the formula of the additional axial force is presented for a parabolical shear lag warping displacement, and the characters of the additional axial force are discussed. For measuring the value of the additional axial force, the ratio of the additional axial stress is proposed. The ratio of a simple supported, a cantilever and a continuous beam are analyzed under a concentrated load and uniformly distributed loads. The results show that the ratio of additional axial stress of a simple supported beam, a cantilever and a continuous beam are usually less than 5%, therefore the additional axial force has little influence on the precision of stress calculation, and the parabolical shear lag warping displacement has high analysis precision and needless to be revised.