Abstract: To study the mechanism of isolated coal body impact instability caused by roadway cutting, the impact rock burst accident in the 32nd mining area of Gucheng Coal Mine was taken as the engineering background. Through the field investigation, numerical simulation and theoretical analysis, the isolated coal body after roadway pressure relief was studied. The stress evolution law is established. The model of the bearing capacity of the isolated coal body after the pressure relief of the roadway and the simplified model of the bearing pressure distribution are established. The mechanism of the overall instability of the isolated coal body in the ultra-deep well is revealed: the stress concentration is in the isolated coal body elastic load zone after the pressure relief of roadway, when the average supporting pressure of the isolated coal body after the concentrated stress and other stresses such as mining activities exceed the impact threshold, the impact is easily induced. Based on this, the criterion of the overall instability impact mechanics of the isolated coal body is proposed. The accident case and numerical simulation calculation verify the correctness of the theoretical calculation. On this basis, specific prevention measures for reducing the average supporting pressure of isolated coal and strengthening the comprehensive compressive strength of surrounding rock of roadway are proposed.