The existing calculation method for the additional axial force (AAF) in wall piers of coupled walls (CWs) requires complex modeling or computing. To solve this problem and to quantify the effect of AAFs on the strength of CWs, a differential equation with respect to AAF was established using a selected coordinate based on the method of continuous connecting links. New analytical formulas of AAF for three types of lateral loads were developed. A simple and practical calculation method for AAF was proposed, which was compared with existing formulas and numerical simulations. The main factors affecting AAF were analyzed using the proposed method. The effect of AAFs on the strength of CWs was investigated using flexural and shear strength indexes. Results show that AAF could be obtained by the proposed method quickly and accurately if the CW geometry and the types of lateral loads were specified, which is simpler than the existing method. The value of AAF for a CW under different types of lateral loads was more affected by the total overturning moment (OTM) than the base shear. The effect of AAFs reduced the shear strength of CWs and the sum of flexural strength of individual wall piers, but the global flexural strength of CWs was almost linearly increased with the increment of AAFs.