Error exists between the internal forces from manual calculation and that from simulation by ABAQUS. By changing line stiffness ratio of girder to beam and magnitude of load, it analyzes how girder’s deflection and its constraint to secondary’s rotation effect the error which is not considered in manual calculation. Also it concludes the facts that the error can’t be ignored since that it may be up to 40%, far larger than the maximum error-5% permitted in engineering. So, it proposed a coefficient table to correct internal forces of beam from manual calculation to promote the safety and economy of structure design.