According to the vehicle loading data collected by the weigh- in- motion system installed on a certain sec-tion of a highway in Guangzhou,including the lane,the motorcycle type,the axle load,the total weight,the velo-city and the vehicle transit time,the time- varying characteristics of vehicle spacing in vehicle flow are analyzed.Then,according to the analysis results,the effect of vehicle load on the bridge is simplified,and,by introducingthe concept of transverse load distribution,the 2D problem of random vehicle load is converted into a 1D one,which helps successfully obtain the stress- time process of the mid- span bending moment of the bridge.By using thestress- time data,a simulation method to obtain the random load spectrum of a highway bridge is proposed,and arandom load spectrum for random fatigue tests (experimental spectrum) is finally compiled.As compared with theexisting simulation method,the proposed method is more effective because it helps to simulate and compile the ran-dom load spectrum that accords better with the actual working conditions of the bridge.