In the fracture study of softening materials,the damage area of crack tip is regarded as a piece of nonlinear crack with cohesive force.The cohesive force has resistant effect on the crack extension.The stress intensity factor of crack solids is a fracture mechanics index that denotes the effect of loading and crack geometry.The resistant SIF of cohesive force has relationship with the values of length of cohesive crack and ultimate tension stress.Through the double-K fracture criterion and fracture model of three point bending beam, the effect of crack cohesive force on the fracture process is analyzed.For maximum loading of bending beam with crack,the calculation approach is presented and the results are well fitted with concrete tested data.