Abstract: Based on the definition of natural bridge frequency, the analytical expression of loaded frequency of a simply-supported beam bridge under train loads was given through solving the vibration differential equation for bridges . The research shows that the loaded frequency of a bridge is in relation to the simplified models of a vehicle, the number of vehicles passing the bridge, train speed, bridge span and so on. The loaded frequency obtained is very proximate to the actual value when a vehicle is simplified as 4 or 2 wheelsets. It varies periodically when the total length of a train is larger than the bridge span. Furthermore, the loaded bridge frequency changeswithtime, relative tothe location ofvehicles on a bridge, and the faster a train moves, the more quickly the frequency varies.