Large mined-out area gives tremendous hidden trouble to the safety of mine's produce,so it is essential to deeply analyze the mechanism of strata failure of mined-out area and farther forecast the strata failure in theory.Firstly,this paper analyzes the stability of mined-out area based on reliability theory,then properly adjusts the values of parameters influencing the stability of mined-out area,more determine the sensitivity of parameters on the reliability index of stability in order to assure the reasonable design in safety and economy,finally,points out that such factors as the ratio of pillar's supporting area and its horizontal transect area,mioing depth and so on have important influence to the stability of mined-out area.