Abstract: According to the main characters of overlap underground chambers, the top pillar between two adjacent underground chambers is simplified to the beam fixed at both ends, and dynamic loading is simplified to harmonic wave. The catastrophic model of double cusp for instability of the top pillar between overlap underground chambers induced by dynamic loading has been established considering deadweight of the beam, and its condition of instability has been worked out. The critical safety thickness of the top pillar has been confirmed according to the instability condition. The influence of amplitude, frequency of dynamic loading on critical safety thickness of top pillar has been analyzed. Taking the Changba Lead-zinc Mine for example, the critical safety thickness of the top pillar between stope and mined-out area below under blasting vibration loadings is confirmed. Research results show that the instability of the top pillar between two overlap underground chambers is not only dependent on the amplitude and frequency of dynamic loads, but also related to deadweight load and characters of surrounding rock. It is accordant to testing results and some related latest research results of blasting seismic effects.