Abstract: When fully mechanized coal mining technology was conducted above the knife gobs in close distance mine seams, due to the inadequate collapse of overlying strata of residual coal pillars, in the action with mining, it could cause the instability of residual pillar in the old goaf of the lower coal and form stress concentration area, and then cause the overlying strata of residual coal pillars irregular collapse damage and the instability of floor strata in mining working face, which threatens the safety in production of mining face. According to the 8# coal seam above knife gobs of Tangshangou Coal Mine, this article researched the working face floor with the force analysis, determined the safety length of working face, and then analyzed the stress of face above the knife gob and the plastic zone of its floor by using UDEC numerical simulation.