On August 31, 2013, a huge rock driven by torrential rain fell on Provincial Highway No. 2 in Badoutzu and nearly hit a passing car. In Badoutzu, the Da-liao Formation is exposed on the rock slope. The strength of the middle part is greater than that of the lower part; therefore, a notch propagates along the lower part during erosion, resulting in rockfall potential at the middle part. To reduce or avoid rockfall hazard, we recommend a method and associated procedures to analyze the mechanics and the influence area of a rockfall event, that is slope unit division, stereographic projection analysis, stability analysis, and rockfall run-out distance estimation using RocFall Software. The proposed method reveals that the event of August 31 was a tensile-failure-induced toppling.