Abstract: In long wall face,by establishing the mechanical model of the thin plate of main roof at a long wall face,one side coal pillar and three sides elastic foundation boundary,and using finite difference method,the paper calculated and studied the distribution characteristics of primary moments and fracture law of main roof. It concludes that the width of the coal pillar Lm and the pillar support coefficient km almost have no influence on the extreme values of pri- mary moments of the long side,short side of the solid coal side and central section,but obviously affect the primary moment of coal pillar side and fracture form of the main roof. Because of the existence of pillar support area,when the thickness h and elastic modulus E of main roof are small and the values of Lm and km are large,the upper surface of main roof in the coal pillar area will not fracture. Under this condition,the fracture sequence of the main roof is that the upper surface of long side advanced coal wall→lower surface of central region or short side advanced coal wall in solid coal side→upper surface in coal pillar area,and the final fracture form is the type of asymmetric “ O-X”. When the values of E and h are large,and the values of Lm and km are small,the upper surface of main roof in coal pillar area will not fracture. Under this condition,the fracture sequence of main roof is that the lower surface of central region→ upper surface of long side advanced coal wall→upper surface of short side advanced coal wall in solid coal side,and the final fracture form is the type of “U-X”.