Abstract: In this paper, the M-N-φ relationship of a T-shaped beam section and the calculation of the externally prestressed tendon抯 deformation with large eccentricities are studied. A general method for analyzing the stress increment of external tendon and flexural behavior of beam is presented. In this method, the action of external tendon is expressed as the equivalent loads applying at the deviators and anchorage ends. The beam excluding external tendon is subjected to bending moment and axial force caused by both applied loads and equivalent loads. The P△ effect is regarded as the forces applying at the nodes of the elements by simplifying the deflection curve into polygon in statically determinate beam. By comparing the different positions of deviators and anchorage ends before and after loading, the force of external tendon is obtained, the deformation compatibility of the external tendon is met, and the loss of tendon's eccentricity is considered automatically. The numerical results agree well with the experimental results.