Abstract: Addressing the existing problem that it is difficult to predict the JCO bending reduction of the large- sized SAWL pipe,theoretic analysis of the JCO forming process is conducted with the elastic-plastic bending theory, resulting in the calculation formulas for bending angle,forming force and bending reduction,etc.,And the accuracy of the mathematic model is verified by relevant experiment. Actual application result demonstrates that with the above mentioned calculation method,formation quality of the large-sized SAWL pipe will be improved.