A frequency domain analysis method based on moving load spectrum was presented for investigating the maximum free vibration response of a bridge.In frequency domain, the Fourier transform was used to obtain the moving load spectrum and the vibration displacement response spectrum of the bridge subjected to moving loads.Corresponding to the maximum value of the moving load amplitude spectrum, the relationship between the dimensionless speed and the axle-span ratio (the ratio of the load wheelbase to the bridge span) was established by the combined use of theoretical derivation and numerical analysis.Then according to the relationship, the speed expression of the maximum free vibration displacement response of the bridge under moving loads was proposed, it is obviously different from the resonance speed.Finally, taking two simply supported girder bridges with different spans as examples, the maximum displacement free vibration response of the bridges was calculated with different ratio of axle span to moving load speed in time-domain, in order to verify the correctness of the results of the frequency domain analysis.The results show that the maximum free vibration response of the bridge can be effectively reflected by moving load spectrum; when the axle-span ratio is 0.72, the maximum response of the bridge is the smallest; when the axle-span ratio is 0 or 1.47, the maximum response of the bridge is the largest; when the moving load travels at the speed according to the speed expression mentioned above, the maximum displacement free vibration response of the bridge will be generated.