Abstract: In order to study the mining width of the coal mining face and the dynamic relationship between the surface crack line and the coal mining face when the ground cracks occurred on the surface above the mining, based on the geological conditions of No. 1409 coal mining face in Huafeng Mine as the background, a mecha nics model of thick and hard overburden strata was established and the criterion of the surface movement cracks occurred was provided.The criterion discussed the rel ationship between the mining width and the max tensile stress of the surface in horizontal direction.With the mining width increased, the max tensile stress of the surfac e would be steadily increased.When the tensile stress reached the tensile strength of the rock, the surfaae cracks of the rock would be occurred along the vertical direct ion of the tensile stress and thus the width of the coal mining face when the cracks occurred with the surface moved and the horizontal distance of the crack line to the coal mining face were obtained .According the calculatio, when the critical width of No. 1409 coal mining face was 127 m, the max tensile stress in a horizontal direction reached the tensile strength of the rock and the actual mining width of the coal mining face would be 150 m.The site measurement showed the cracks occurred at 398 m of the gateway to the coal mining face and the predicted value was at 410 m of the mine roadway.Thus the predicted value and the actual measured value were basic ally the same.