To find the reason of remarkable vibrations of the protection-frame of a intermal-combustion engine fork-lifter while working at a idle speed,avibration analysis was made in this paper. Firstly, the natural frequency and modeshape of the fork lifter were acquired in a modal test. Secondly, the mode acquired by the finite element analysis were torsion around z axle while its frequency was 32.7Hz, compound mode of torsion around z axle and bend around z axle while its frequency was 81 Hz, which was consistent with the tested mode of 33．7Hz and 80.6Hz respectively. The exciting frequency from the engine running at idle speed is between 25 and 26 Hz．So, the protection-frame vibration is forced vibration but not resonance, which supplies references to the vibration is olation design.