The jump and pitching vibration occurred when a wheel loader is driven in the straight-line at a specific speed， accompanied by the jump vibration of the seat system. To identify and study the key influence factor of the abnormal traveling vibration， a four-degree-of-freedom vibration system model of the loader is established， based on the vibration characteristics， and the natural frequencies and corresponding modal eigenvectors are obtained by state space method. In order to verify the theoretical vibration model， the operational modal analysis （OMA） test is carried out under the abnormal vibration condition. By the OMA， the vibration modals of the loader and the seat system are successfully identified. The modal information is compared and the results show that the deviation of natural frequency between the theoretical and the OMA is less than 5%， and the calculated eigenvectors are consistent with the OMA mode shapes. The validity and accuracy of the theoretical model are verified. The tire vertical stiffness is identified as one of the key parameters based on the theoretical model， and its influence on the natural frequencies of jump and pitch vibrations of the loader is studied.