Based on finite element analysis of thermal mechanical behavior, structural optimization design was proposed for a side cooling collimating mirror subjected to high heat load for a beamline at SSRF(Shanghai Synchrotron Radiation Facility). The temperature distribution,stress concentration effect, maximum equivalent(vonMises) stress, and slope error of the mirror were analyzed.In particular, the cooling water channels of the traditional structural design were optimized, and the modified designs were further optimized. Although the traditional structural and the improved designs could meet requirements for the temperature and thermal stress, the deformation gradients were relatively large for several structural designs, and this led to larger slope error. The further improved structural designs could be of better performance. Based on finite element analysis of thermal mechanical behavior, structural optimization design was proposed for a side cooling collimating mirror subjected to high heat load for a beamline at SSRF (Shanghai Synchrotron Radiation Facility). The temperature distribution, stress concentration effect, maximum equivalent (von Mises ) the cooling water channels of the traditional structural design were optimized, and the modified designs were optimized. thermal stress, the deformation gradients were relatively large for several structural designs, and this led to over slope error. The further improved structural designs could be of better performance.