It is known that coupled bending and torsional effect is a distinct characteristic of curved girders. According to mechanical characteristics of box girders with corrugated steel webs and theory of box girders, warping normal stress of box girders with corrugated steel webs caused by Torsion Loading is studied. On the basis of second theory of Wu, restrained torsion differential equation of box-girders with corrugated steel webs is proposed. Corrugated webs are assumed as orthotropic plates. Constraint torsional normal stress and shear stress are deduced using Initial parameters method. The results demonstrate that the method is more accurate compared with conventional ones, and it is connected with existing bridge calculation theory. The method is then a practical one in this respect.