Abstract: According to the evolution of fatigue cracks in welded structures, the fatigue life of welded structures was defined as the sum of crack initiation life Ni and crack propagation life N_p, and the SN curve and fracture mechanics combined with fatigue life evaluating method was proposed. Equivalent structural stress method and the lower 99% boundary of master SN curve were utilized to evaluate the crack initiation life, and cracks at the end of the initiation stage was regarded as semi-elliptical surface crack. Paris Equation and the stress intensity factor range of semi-elliptical surface cracks were utilized to evaluate the crack propagation life. Fatigue test data of 5 crane runway girder specimens from a reference were utilized to the comparisons and Validation investigation. Result showed that equivalent structural stress can characterize the crack initiation state of complex welded structures well, and there is a good consistency between the calculating fatigue life and the test life.