Dynamic failure of brittle rock controlled by complex tensile and shear loads has been a hot topic in both research and practice field. Based on plasticity and damage theory of continuum mechanics, a new Elastoplastic Damage Model (EDM) combining compressive/tensile and shear failure modes together for brittle rock materials is proposed in this study. The EDM is further explicitly solved by Finite Difference Method (FDM) and implemented into FLAC3D commercial software to simulate rock problems. One of application problems of rock mechanics is analysis of the stability of large section tunnels. Ganggou Tunnel is a typical super-large section tunnel with the cross area up to 219.78 m2. The stability of Ganggou Tunnel during excavation process has become a hot issue. Considering the geological condition of Ganggou Tunnel, sets of numerical cases of tunnel excavation with variable net distances between tunnels are carried out. Numerical result such as the distribution law of plastic failure zone, displacement deformation and stress evolution in surrounding rock are analysed. Results indicate that plastic failure zone of surrounding rock increases as net distance between tunnels decreases. While both eventual displacement deformation and tensile stress after tunnel excavation decrease as net distance increases. The new model is validated by well describing the transitional mechanism of tensile and shear modes for rock dynamic problems. The proposed model will provide an effective numerical tool for studying the stability of rock in similar tunnel projects.