In practical tunnel construction, the stress redistribution of underground rocks caused by tunneling usually leads to microcrack-expansion damage in the surrounding rock, with accompanied plastic flow deformation. The high pore water pressure is generated in the pores of rock and microcracks under stressed conditions in groundwater environment, which in turn influences the mechanical properties and the changes of the failure modes of the surrounding rock. To investigate the coupling effect of two types of failure mechanisms which are the damage-induced stiffness degradation and the plasticity-driven flow. based on the elastoplastic and damage theories, the modified effective stress principle is adopted to consider the effects of pore water pressure, and then an elastoplastic damage constitutive model is developed adopting the Drucker-Prager yield criterion. To implement this model, a numerical integration algorithm-implicit return mapping algorithm is developed with considering the effect of pore water pressure, and the detail descriptions of the implicit return mapping algorithm about the trial stress returning to the smooth cone surface or sharp point singularity on the yield surface are given. The implicit return mapping algorithm has good performance with regard to stability and accuracy. The set of parameters related to elastoplastic damage models is large and not easily determined. To resolve this issue, a back analysis method is introduced. The elastoplastic damage constitutive model is implemented by adopting the method of object-oriented programming using C++ language. The proposed model and the computational procedure are validated by comparing the numerical simulations to the experimental measurements. The proposed procedure is applied to Jilin Fusong tunnel project to analyze the development of the plastic zone and the damage zone. The results show that the proposed elastoplastic damage constitutive model can well describe the mechanical behavior of the rock, and the computational procedure can simulate the practical engineering problems, giving certain guidance for site construction.