Pillar-shaped rock masses are widely developed throughout the world with some standing firm for thousands of years. Unstable pillar-shaped rock masses display different failure modes and it is often difficult to determine in advance the specific failure mode of a complex rock mass. For example, the displacement deformation of the Jianchuandong rock mass (JRM) in the Three Gorges Reservoir Area has been about 20 mm over the last 2 years. With a volume of 360 000 m3, this unstable pillar-shaped rock mass stands on the bank of the Yangtze River's Wu Gorge, threatening the safety of the waterway and the residents of neighbouring areas. Developing Hungr's quantitative discriminant on the rock collapse and rock slump modes, several cases are used to verify its validity. The failure mode of the JRM is judged to be rock collapse. This judgement matches well with the actual field survey and monitoring data. For pillar-shaped unstable rock masses with a rock collapse failure mode, this paper develops a new slope stability calculation mode by deeming the intact pillar-shaped rock mass to be the load. The result of such a stability calculation shows that the JRM is basically stable at present, but the safety margin is extremely narrow. Prevention work should be carried out as soon as possible for better results at a lower investment before the JRM experiences large-scale deformation and failure.