Abstract: Taking mining status of Tangshangou coal mine as an example,the expansion and closure process of the overlying strata under gravity stress field and seepage field in knife column gob are analyzed by using UDEC.The data shows that the " V" shape of fractured zone has been formed by mining impact of roof rock and supporting pressure of gob pillar in the advancing process of working face.The generated fissures will be communicated to the upper coal goaf by inverted trapezoidal,and the water flowing fractured zone rapidly expand to form conductor.