Abstract: Taking the 2204 working face of Xinzhi Colliery in Huozhou as the background, through experiment of analog simulation, analyze the structure type of old goaf overburden rock breakage, instability activation mechanism of overburden rock and instability form and condition for stability of voussoir beam of old goaf. As the result, after mining and breakage, the rock mass structure of old goaf and overburden rock can be divided four types. This secondary rock structure has an important effect on old goaf activation. The existence form will decide the basic characteristic of old goaf activation. Through the mechanics analysis on voussoir beam of old goaf, it is obtained that instability due to sliding mainly depend upon the physical mechanics characteristic of key block and sideward rock, W/T value, and additional load of building on surface and so on. Rotational instability is related to the physical mechanics characteristic of key block and sideward rock and thick and length ratio h/l of the hanging rock.