Groundwater inflow during tunnel excavation and its influences on the hydro-geological environment (HGE) are vital to green construction and environmental sustainability. To evaluate these issues in completely weathered granite formation, a water inrush event occurred in Junchang tunnel is introduced to investigate the changes in HGE including groundwater level, ground surface settlement, surface fractures as well as the tunnel settlement. Then, a seepage-erosion coupling model is developed to predict water inrush and its influences on the HGE changes. This model can describe the characteristics of nonlinear seepage and particle erosion for water inrush and its effectiveness is verified by the field results. Using this model, the potential water inrush and the changes in HGE are evaluated for future excavation in various high-risk positions. Consequently, the real-time water inrush process, the HGE changes and the practical implications are discussed with the field and numerical results. The main research findings include: (1) the evolution direction, mutation time and the volume of the water inrush, and (2) the influenced region, evolution time and the extend of the changes in HGE. These findings can provide references for the early warning, monitoring and prevention for future constructions.