The modal analysis has been done to the problem of the rupture of the bolt in the pump which used in meeting an emergency in the transfer case of certain special vehicle. At first, the modal experiment of the transfer case has been done, and the modal parameters have been obtained, and then let the vehicle steer on the road which was similar to the condition of the working station. Combined with transmission route of stimulation, the vibration of the vehicle was analyzed. The correlation analysis of mode shape was done so as to prevent the generation of spurious mode and mode from missing. The fundamentality reason of rupture of the bolt was found, the corresponding ameliorable method was given. 