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The 3 August 2014 Ludian, China, Ms = 6.5 earthquake caused many large landslides. The biggest occurred at Hongshiyan near the epicenter, had a volume of 1.0 × 107 m3 and dammed the Niulanjiang River, creating a large lake. Post-event field investigations yielded detailed data on the following aspects: rock structure of the landslide, the local lithology and geometry of the area around the landslide dam and composition and grain sizes of the debris avalanche. Based on these data, this work analyzes the geology and topography of the Hongshiyan area, and explores reasons for the occurrence of an unusually large landslide at this location. Our analysis suggests the following conditions are responsible for this catastrophic event. (1) Due to recent crustal deformation, intense incision on the river increased topographic relief with steep slopes and scarps. (2) Combined structures, including unloading fissures, high-angle joints and low-angle beds along the river, as well as an upper-strong and lower-weak interlayer structure on the slope, especially the existence of weak layers in the slope, are important factors that contribute to this large failure. (3) Hongshiyan lies near an active fault, where intense crustal deformation has resulted in rock fractures and weathering, and frequent earthquakes may progressively reduce the strength of the slope. (4) During the Ms = 6.5 earthquake, the terrain and site conditions led to abnormally strong ground shaking. The combined impacts of these factors triggered a very large landslide during a moderate-sized earthquake.