A new model for a random vibration analysis of vehicle-bridge systems subjected to road roughness excitation is proposed. The bridge is modeled as beam elements and the vehicle is modeled as two-degrees of freedom mass-spring-damper system. The load caused by deck irregularity is equivalent to pseudo excitation load. The model is solved with pseudo excitation method(PEM) following by mode superposition iterative solution introduced. The validity of model and algorithm has been verified through the result of Monte-Carlo method. Taking a simple beam bridge as an example, the random vibration response of vehicle-bridge coupling caused by road roughness and vehicle speed in frequency domain proves the following conclusion. The root mean square values (RMSV) of vertical displacement at mid-span varies with the vehicle speed obviously, and the first frequency peak and band width of displacement and acceleration power spectrum density (PSD) is changed with the vehicle speed. The peak, frequency and band width of acceleration PSD adjoining the bridge bearing change with the vehicle speed obviously. The vibration response of vehicle in vehicle-bridge coupling system caused by deck irregularity varies with the vehicle speed obviously. The effect of vehicle speed on the random responses caused by deck irregularity cannot be neglected when the vehicle-bridge coupling system is researched.