With the increase of global extreme weather and human engineering activities, the degradation of the surrounding environment along mountainous railways may lead to low-frequency debris flows entering an active period, posing a serious threat to railway operation and maintenance safety. This study takes a debris flow adjacent to railway in Baise City, Guangxi as the research subject. Based on field investigations, historical remote sensing images, and other methods, the study analyzed the source materials classification and development characteristics of debris flows in mountainous railway areas, and investigated the watershed environmental evolution and degradation of this debris flow. Using theoretical calculation methods, the study back calculated the debris flows under current rainfall conditions, obtaining dynamic parameters such as peak storm discharge, peak debris flow discharge, and quantity of eroded solid material, quantitatively revealing the impacts of factors such as deforestation, road construction, and wildfires on the disaster-causing capability of debris flows. Lastly, a detailed discussion on the disaster-causing factors of this debris flow was provided, revealing the disaster-causing mechanism of debris flows in this mountainous railway area, and proposing corresponding prevention and control engineering recommendations. The findings can serve as a reference basis for disaster prevention, risk prediction, and control of debris flows in mountainous railway areas.