Based on the secondary development platform of OpenSEES software, this paper studied the restoring force model of unbonded partially prestressed concrete beam sections. According to the statistics of experiments and theoretical analysis, the moment-curvature hysteretic model of the section for unbonded partially prestressed concrete beams was established, and planted into Material class in the software of OpenSEES by compiling Visual C++ programs, which is used for the nonlinear analysis of unbonded partially prestressed concrete structure without iterative calculation of stress increment for unbonded prestressed tendon. The crucial problem of stress analysis for unbonded partially prestressed concrete structure was then solved. Moreover, under displacement control, this paper considered the nonlinear beam-column element based on flexibility to calculate the moment-deflection hysteretic curves of unbonded partially prestressed concrete beams under low cyclic reversed loading on the basis of established restoring force model. The calculation results are reasonably identical to experiment results, which indicates the reliability of established restoring force model, and provides theoretical basis for the nonlinear analysis of unbonded partially prestressed concrete structures.