Abstract: Based on the beam theory,the relation between the bending moment and the measured lateral deformation of diaphragm walls could be established.The author introduced the detailed procedure to calculate bending moment by this method.With this method the engineers could know the bending moment during the pit excavation and take measures to avoid the diaphragm wall rupture if the bending moment achieved to the limit value.More,with the application of this method in deep foundation pit,the field measurements of stress of reinforcement could be canceled and the total expense would be reduced.