Grating shaft of a double grating spectrometer is an important part which is used to fix and drive gratings, deformation and vibration of which both have effects on final measurement result. According to operating principle and working condition of the double grating spectrometer, centroid adjustment, light weight, avoid resonance and the decrease of the random vibration were chosen to be main goals in the optimization. First, three-dimension model of concave gratings, grating shaft and its fixture were built in UG. Then, they were imported into ANSYS Workbench, modal analysis and random vibration analysis were done. Finally, multipurpose optimization was done according to the result in last step. After the multipurpose optimization, centroid of the rotating parts was adjusted to the rotating axis. Its mass reduced from 0.606 30 kg to 0.539 43 kg. Its second natural frequency increased from 184.83 Hz to 187.77 Hz, and the maximal random vibration deformation on Z axis of concave gratings reduced from 33.394 ?滋m to 27.147 ?滋m. Centroid adjustment of structure could be realized directly by common finite element analysis software, the author achieved this goal when building three-dimensional models, and guaranteed that centroid of the rotating parts moved only on their rotating axis in optimization, so that centroid adjustment and light weight can be realized at the same time, which can be widely used in many other projects.