Abstract: It can be produced warping normal stress and shear stress for a steel beam constrained at the both ends under an external torsion. It is very complicated to use the traditional treatment for sectorial coordinate and bi-moment. Thusly, the treatment is certain blindness for many engineers in the design of considering the steel girder torsion effect. Through the characteristics of the restrained torsion stress and deformation, a new simply method of the calculation of the beam under restrained torsion was brought, based on the compatibility of the deformation. It can well calculated the steel beam under a external torque. The results show that this simple calculation method has good precision and accuracy, and it can be used for engineering design and theoretical calculations.