Strengthening method of using external pre-stressed steels has many advantages. It gets the recognition of researchers and is widely used in the strengthening and rebuilding projects. However, calculation theories about the method are still lagging behind the practice. It is difficult to put the theories about unbounded pre- stressed steels into construction of external pre-stressed projects because of cross-section deformation incoordination and quadratic effect. This paper takes the design theory and method of external pre-stressed beam in normal use stage as the main line, uses the iterative method by the relationship of equivalent load- deformation-stress increment, numerical simulation the deflection and reverse arch value of the beam. On the basis of the influence of quadratic effect, the equivalent reduction factor of the external pre-stressed steels and whether steels being unloaded or not, a simplified calculation formula for the beam rigidity at using stage is proposed. By compared with the experimental dates, this method is verified feasible in practical application.