The under building Dreamline at Shanghai Synchrotron Radiation Facility (SSRF) is a wide range and high resolution soft X-ray beamline. There are many difficulties to overcome for building a beamline with such high target. Especially, the thermal deformation of optical elements is one of the main obstacles. Based on the focal parameters of SSRF, the thermal power density distribution absorbed by optical elements is calculated. Finite element analysis is utilized to perform the thermo-mechanical analysis of M1 and M2 to obtain the temperature gradient distribution, the surface displacements and the slope errors of these optical elements, and then ray tracing simulation is done. In addition, a thermal deformation correction device is designed to cancel the deleterious effects of these deformations. According to the result, even under the worst-case thermal deformation of the optical element surfaces, effective cooling and appropriate focusing corrections are able to cancel the deleterious effects of these deformation.