Abstract: According to distribution condition of joint fissures of top coal in front of fully mechanized coal face of one coal mine, a numerical calculation model of fully mechanized coal face was built, UDEC software was used to make numerical simulation for fully mechanized coal face with different mining heights under condition of same geology, and deformation features of coal and rock in front of fully mechanized coal face and force condition of top coal were analyzed. The research result showed that reasonable mining height is important factor to ensure stability of roof.