Abstract: A fatigue reliability assessment method is developed, based on the combination of linear elastic fracture mechanics and long-term monitoring data, and the application research is presented with the object of welded details for Runyang Bridges. The probabilistic models of the fatigue loading effects are obtained by the basis of the long-term monitoring data first. Then the expression of the fracture resistance is derived. And the effect law of the initial and critical crack depth on the fracture resistance is analyzed. On this basis, the random numerical simulation is employed to obtain the probabilistic model of fracture resistance. At last the time-variant law of the fatigue reliability is studied in detail with the consideration of traffic growth. The results show that fracture resistance follows a lognormal distribution and is very sensitive to initial crack depth. By the consideration of the traffic growth, the fatigue reliability of welded details will be less than the level of the target reliability when the service time does not reach the design reference period.