When testing tracking accuracy of the optoelectronic tracking terminal by optical dynamic target, in view of the problem that surface-shape error of the rotating photoelectric collimator caused by the centrifugal field is too large, a method based on flexible beam is proposed to reduce the surface-shape error of primary mirror of the rotating collimator under centrifugal condition. Firstly, the optimal radial position of the supporting point of the primary mirror is solved; Secondly, the key design parameters of the flexible supported beam on the back of the primary mirror are optimized on the Isight optimization platform; Finally, Sigfit software is adopted to analyze the surface-shape error under centrifugal condition. After adopting the optimized scheme of the backside support, under the influence of 1.64g centrifugal force, the surface accuracy RMS of primary mirror is better than 0.03λ (λ= 632.8nm), and the first-order natural module frequency of the collimator is 167Hz. The 4D interferometer is adopted to detect the wave aberration of the collimator, the result indicated that?the RMS value of wavefront aberration of collimator is better than 0.067λ. Through simulation analysis and testing experiments, it is proved that the dynamic stiffness and surface shape accuracy of the rotating collimator with flexible support meet the application requirements of optical dynamic targets well.