Abstract: The stress and deflection of the continuous main-beam in a light steel structure floors are studied,by means of structure test methods and theory a three-span method.The actual deflection formulas for the middle section of a three-span continuous steel beam are obtained.Making use of standard FEM programs,the theoretical and testing formulas are verified.The causes of differences by two researching methods are also revealed.This research has valuable information on designing new structures and supervising old structures in service.