Abstract: The blasting excavation is an important influencong factor for the instability of engineering rock mass. In view of the characteristics of combined mining for open-pit and underground, catastrophe theory models of the underground chamber roof instability induced by open blasting disturbance are built by taking the underground chamber roof as a horizontal simply supported beam based on the non-linear theory. The instability criteria and the critical safe thickness of the underground chamber roof are derived according to the catastrophe theory models. At the same time, the important influencing factors for the critical safe thickness of the underground chamber roof, such as the amplitude and the key frequency of the blasting vibration, and the characteristics of the surrounding rock are discussed. The critical safe thickness of the roof between the open-pit and underground stopes is obtained by a safe thickness analysis of Changba Pb-Zn deposit group mined-out area roofs in Gansu province as an example. It is shown that the dynamic instability of the underground chamber roof is obviously nonlinear and discontinuous, while the critical safe thickness of the underground chambers roof not only depends on engineering geological characteristics of the roof, but also on other multiple factors, for example, the amplitude and the key frequency of the blasting disturbance, as is consistent with the actual situation.