Prestressed corrugated steel-concrete (PCSC) composite girders are often subjected to combined bending and torsion in practice, but the existing theoretical models have mainly focused on pure torsional behavior. Thus, this paper proposed a modified combined action softened truss model (MCA-STM) to predict the flexural-torsional coupling behavior and failure modes of PCSC composite girders. The equivalent loading and nonlinear equations were modified, and the convergence criteria of flexural and torsional failure were established, considering the structural characteristics of PCSC composite girders. A new optimized algorithm was employed to solve the MCA-STM, providing a more efficient and numerically stable solution procedure. A 3D finite element analysis (FEA) model of the PCSC composite girder was established, which was verified by comparing the available test results. A parametric study was carried out based on the verified FEA model to get more datasets with various parameters, and the results were compared to the proposed analytical model. The curves obtained by the FEA and MCA-STM are in good agreement, indicating that the proposed analytical model can predict the full flexural-torsional coupling behavior and failure modes of the PCSC composite girders.