Abstract: For the characteristics that the overburden of the stope in some districts has thick and hard rock formation, the blockness of broken rock mass is big, resulting in the distortion of the result of the breaking span calculation formula. By considering various factors such as interaction of the rocks and the support parameters, using combined deformation in material mechanics, the thick and hard rock formation period breaking span mechanical model is established, the period breaking span calculation formula is obtained, the derivation formula is proved by the on-site measurement data and compared with the traditional boom-type breaking span calculation formula.The research result shows that as the thickness of the rock formation increases, the deviation of the result of the traditional calculation formula to the actual value is increasingly apparent, while the result of the derived span calculation formula well coincides with the actual value. Therefore, the derivation formula has higher reliability and reasonability. The factors influencing the period breaking span of the rock beam are: thickness of the rock formation, tensile strength, sinking amount of the broken rock mass, carrying load, support force transferred to the fundamental roof, and contact length of the non-broken rock beam between the direct roof and the fundamental roof. The thickness of the rock formation, the tensile strength and the carrying load are main factors. The breaking span increases along with the increase of the thickness of the rock formation and the tensile strength; and moreover, as the thickness of the rock formation increases and the breaking span increases, the formed lumpiness after the breaking increases. The rotating sinking amount of the broken rock mass has little influence on the breaking span; and the carrying load is in a negative correlation with the breaking span. When the load value is small, the influence on the breaking span is large. When the support parameters R and p increase, the breaking span increases but increasing amplitude is limited, which indicates that the support cannot effectively postpone the occurrence time of the fundamental roof or cannot prevent the rotating sinking.