Based on the cold roll forming steel, its characteristic of roll-forming and deforming rule was analyzed. The analytic model has been built by using the theory of Vlasov open thin wall beam restraint distortion. Through studying the model, and by using simplified analysis method, the whole distortion of cold roll forming course could be divided into flange girth torsion deforming of longitudinal direction and circular bead rebounding deforming of lateral direction. The energy method has been adopted to calculate its longitudinal direction and lateral direction deforming energy during the roll-forming course respectively. The flange girth bending angle expression of the whole cold roll-forming steel was obtained. Theory curve was compared with the relative experimental and the existing conclusion curve, the availability, dependability of the calculating method and validity of the calculating result are validated.