Abstract: Researching into the performance deficiencies about vibrations of driver seat，while fork-lift truck is driving， and analyzing the influencing factor based on the multi-rigid-body dynamic method with considering the influence of flexibility of fork-lift frame and road on the dynamic characteristics of. A new rigid-flexible coupling dynamic model of fork-lift truck is established. Dynamic characteristics of the new established system are studied through simulating the system under different driving conditions. The theoretical model and the simulation results are validated through experiments. And then, for melioration of the performances of the fork-lift truck vertical vibration, improvement measures of complete engineering machines for decreasing the vertical vibration of seat is offered as following：The shaft from the steering bridge is used in the ellipse, and the elastic hinge is used by the rubber cushion block in the support; the overhead guard is connected with the frame by the rubber cushion blocks at four different locations, and the motor cover is hinged with the overhead guard, while the seat is fixed on the motor cover; the engine is fixed on the frame by the rubber cushion blocks (shock mount)in two different position. And then，for melioration of the performances of the fork-lift truck vertical vibration，some technical measures are put forward on the basis of simulation，which is validated by experiments under the same conditions. The results show that after the forklift vehicle system structure improving, the fundamental frequency of seat is lower and vibration energy is decreasing. The result proves the practical value of this method in the research of the vibration characteristics of complete engineering machine.

Key words: Finite element method, Fork-lift truck, Rigid-flexible coupling, Vibration characteristics