Abstract: Aiming at the problem that floor impact happened frequently in roadway under the action of tectonic stress of fold region,based on mechanical conditions occurring in the floor,reasons for the floor impact in roadway under the action of horizontal tectonic stress were analyzed by theory of elastic thin plate,mechanical model of unstable failure of floor was established,expression formula of critical load on unstable failure of floor was calculated by theoretical derivation,calculation formula of horizontal stress endured by floor was given and danger coefficient of floor burst was modified with the upper longwall(LW) 250206 in Yanbei colliery as the engineering background.