The principle of electromagnetic launch is introduced. The equation of electromagnetic load applied on rails is given. Based on the calculated result of electromagnetic pressure, the equation for calculating bending stress of rail on the elastic foundation is deduced using superposition principle. The stress distribution in rail with the armature in different positions is analyzed. Compared with results considering the force on the rail as uniformed load, the calculating results of stress distribution are more accuracy and useful for intensity and rigidity design of electromagnetic launch. The study may provide a theoretical basis for further analysis of systemic mechanical properties.