Taking the G3 Tongling road-rail bridge as the research object, a finite element calculation model of bridge was established, in order to investigate the dynamic response of the bridge structure and the train on the bridge when the cables were fracturing under extreme action in long span road-rail cable-stayed suspension bridge. Firstly, the dynamic response of the bridge structure after stayed cables or suspender cable fracture was studied. Then, the dynamic response of the bridge and the train under the cable fracture conditions of the train-bridge coupling vibration was studied. Results showed that the cable-stayed suspension bridge had high structural stiffness and safety redundancy. After a single stayed cable or suspender cable fractured, the train could be maintained. Continuous cable fracture might occur when more than four cables were fracturing in one side of the suspender cables area of the bridge. Continuous cable fracture would not occur when eight cables were fracturing in one side of the stayed and suspender cables alternating region and the stayed cables region. Setting the dynamic amplification factor of cable fracture equal to 2.0 was reasonable, but they had engineering application value only for the remaining suspender cables and stayed cables near the fracture cables. When the cable was fracturing, if the train was passing by, the vertical acceleration of the train would change abruptly, but the acceleration was within the safe range. The stress increase of residual stayed cables and suspender cables was mainly due to the action of main girder, and the proportion of train action was less than 13%.