Abstract: Aimed at the problem that there is serious fatigue cracking in the steel box girder of a suspension bridge, the vehicle type, wheelbase, axle load, total weight and overload of the bridge are statistically analyzed, and the traffic load characteristics of the bridge and the difference of random traffic flow in each lane are clarified by the base of the data collected by a WIM dynamic weighing system. According to the dynamic strain monitoring data of a real bridge, the fatigue stress spectrum of each lane under operating condition was obtained by using the rain-flow counting method and the Palmgren-Miner linear damage accumulation theory. Based on the UD-RBF-IMC algorithm, the fatigue reliability of U-rib butt weld was evaluated by linear elastic fracture mechanics. The influence of traffic volume and the shaft weight on fatigue reliability were studied. The result show that:the type of a fatigue vehicle can be simplified to V2-V10, a total of 9 categories, the total weights of V2 vehicles are a unimodal skewed distribution, overload rate of less than 4%, the total weights of V3-V10 vehicles are multi-peak distribution, overload rate greater than 30%, up to 69%. The proportion of V2-V10 vehicles of a heavy lane is obviously higher than that of other lanes. The daily variation of temperature has little effect on the fatigue stress spectrum, and the influence of sampling frequency on the stress spectrum is significant, which should not be less than 50 Hz. Combining the respective advantages of UD, RBF and IMC, the accuracy and efficiency of the fatigue reliability index of steel box girders based on monitoring data are effectively improved. The effect of axle weight growth coefficients on fatigue reliability is obviously greater than that of traffic volume growth. In addition to controlling the traffic volume during operation, it is also necessary to focus on controlling the proportion of heavy vehicles and overloading rate. When the growth coefficient of traffic volume is 3% and the growth coefficient of axle load is 0.6%, the fatigue life of measuring point 1# is only 74 years. The number of overloaded heavy-duty trucks of passing lane is less, the high-level stress cycles are less, and the fatigue life is longer, while the number of heavy-haul trucks of a fast lane and a heavy lane is more, the high-level stress cycles are more, there is a risk of fatigue cracking, which should be paid more attention to.