Abstract: The collapse of large unstable rockmass fails suddenly with a short initial failure process. Therefore, it is different to make in-situ observation and obtain valid data timely. As a result, a centrifuge model test is introduced. In this paper the unstable rockmass at the Zengziyan cliff is taken as a case, and the collapse mechanism for this tower-shaped unstable rockmass is examined. The key influencing factors are analyzed quantitatively. Centrifugal acceleration of the Zengziyan model is 73g at failure moment when the compressive strength of the bottom zone of the unstable rockmass model is 60 kPa. The centrifugal acceleration is just 18g at the failure moment when the compressive strength of bottom zone is reduced to 40 kPa. The strength of the bottom zone can control the whole stability of the unstable rockmass, which is proved with the contrast tests. The failure mode of the middle and upper part of the high-steep rockmass model is different from the original one, and the reason is that the exogenic action is introduced due to the test factors. The other reason is the model material without considering joints and tensile strength. The results show that the collapse mode of the unstable rockmass like Zengziyan is controlled by the rockmass strength at the bottom and the properties of the overlying rockmass. These results provide experimental basis and data support for the failure mechanism of this kind of unstable rockmass, and they are also an important reference for collapse mechanism research.