Bagong Dam is located in Trenggalek Regency, a dam with a diversion tunnel. Although engineering geological investigations with general descriptions have been carried out at the Dam site, the stability of the tunnel has not been analyzed numerically. The purpose of this study was to determine the tunnel excavation method, and the tunnel support system using the JSCE classification and to analyze the stability of the tunnel using the Finite Element Method (FEM). A core assessment was carried out to determine the quality of the rock mass on the tunnel trajectory. The physical and mechanical properties of intact rocks from core samples were obtained through laboratory testing. The subsurface rock layers in the Bagong Dam diversion tunnel consist of andesite breccia from fair to good, clay sandstone from poor to good, and then poor-quality limestone. Based on the JSCE classification, the excavation method used is the full-face method. The support system used is 10 cm thick sprayed concrete and 4 m long rock bolts with a distance variation of 1.5 m installed on the roof and walls. The results from the stability analysis of the diversion tunnel’s outlet side show that, prior to applying the earthquake load, the Safety Reduction Factor (SRF) was 3.52, which is significantly greater than the safety threshold of 1.5. After the earthquake load was applied, the SRF decreased to 3.36, still exceeding the minimum threshold of 1.1. It can be concluded that the outlet side of the tunnel remains safe and stable.