The external prestressed tendon is regarded as a constant strain member consolidated and connected with the beam at the anchorages and the deviators，respectively. Based on the geometric relationship of the external prestressed tendon and beam body at the anchorages and the deviators， the tangent stiffness matrix and resistance of the tendon end in the structural coordinate system are derived by the co-rotational （CR） method. Combined with the conventional CR plane beam element， a geometric nonlinear finite element model is established and the program is compiled. The geometric nonlinear analysis of the example in two cases is carried out and compared with results obtained by ABAQUS. The results show that the beam-ends horizontal displacement， the middle-span vertical displacement and the axial force of the external tendon calculated by the two methods are in good agreement， for both prestress application state and load application state. For the axial force of the external tendon， the proposed method can better reflect the mechanical performance of the external tendon as a constant strain member， which can be used for the nonlinear finite element analysis of the externally prestressed structure.