Multidisciplinary and multi-objective optimization design method was applied in the researches of collimator mirror lightweight of wide field spectrometer of extremely large telescope. Lightweight design was operated independently on the basis of collaborative optimization which is one of the best feasible multidisciplinary design optimization approaches combined with multi-objective genetic algorithm to carry out the optimization, and the Pareto optimal set was obtained. The lightweight performance with different lightweight holes and the thermal stability of light-weighting collimator mirrors were comparatively studied. With triangle lightweight hole of collimation mirror, the light weighting rate is about 70% and the peak-valley (PV) value is 82.696 nm. With rectangle lightweight hole of collimation mirror, the light weighting rate is about 75.3% and the PV value is 107.03 nm. When the temperature of the collimator mirror changes about 10 K, the deformation of the collimator mirror is added doubled. The results indicate that the comprehensive evaluation of triangle lightweight hole is better than rectangle holes; the coupling and interdisciplinary relationships among the lightweight, the lightweight hole shape parameter and the deformation of mirror are comprehensively considered during the multi-objective optimization of collimator mirror lightweight structure. Thus the designers can choose the satisfactory optimization results according to their demands, so as to significantly reduce the product development cycle and costs.