In this paper, a deterministic approach is adopted to estimate the critical rainfall threshold of a rainfall-induced landslide. The critical rainfall threshold is defined as the accumulated rainfall when the safety factor of the slope is equal to 1.0. The process of deterministic approach first establishes the hydrogeological conceptual model of the slope based on a series of in-situ investigations and laboratory experimental results. Second, the hydraulic and mechanical parameters of the model are calibrated with the long-term monitoring data. Furthermore, the critical rainfall threshold of the slope can be estimated from the relational analysis between the accumulated rainfall and the slope stability. Finally, the critical rainfall threshold of the slope can be obtained through the integrated analysis of rainfall, infiltration, seepage, and slope stability. Taking the slope located at 50k+650 on Tainan County Road 174 as an example, the results show that the critical rainfall threshold of the study case is around 640 mm. Our preliminary results appear to be useful for rainfall-induced landslide hazard assessments. The findings can also serve as a good reference for establishing an early warning system for landslides.