Abstract: A new flexible support system was designed for the rectangular collimator of extremely large telescope. The system was a side support structure supported by six hollow cylindrical bars. The structural parameters and position parameters of flexible support system were optimized by multi-objective optimization algorithm, and the stability of the optimized flexible support system was also studied. The optimized flexible support system for collimator had a maximum peak valley (PV) value of 168.23 nm, root-mean-square (RMS) value of 30.306 nm and a weight of 229.21 kg under the influence of gravity only, which satisfied the design requirements. When the ambient temperature changed within 20℃~23.7℃, the mirror performance would not be affected; besides, random ground vibration would not affect the performance of the collimator and destroy the support structure. The results show that the multi-objective optimization design of the flexible support mechanism for collimating mirror comprehensively considers the coupling problems between the shape parameters of the flexible support mechanism for collimating mirror, the safety and reliability of the mechanism and the deformation of the optical surface, etc. Designers can choose the most satisfactory solution on the global scale as needed, which will greatly reduce the development cost and cycle time.