Abstract:The distinct element method,physical simulation and theoretical analysis have made it clear that the bolted stratified rock roof forms combined voussoir beams. The combined voussoir beams effect is one of the working mechanics of the rock bolting. Based on this theory,the average stress formula of the combined voussoir beams is derived. This formula can reflect the influencing factors on the stability of rock roof synthetically,such as the thickness of combined voussoir beams,the span of roadway,the thickness of single rock stratum,and the integrated coefficient of rock. The influencing regularity is discussed and the results complies with the practice.