FORMOSAT-5 is the first space program that National Space Organization (NSPO) takes full responsibility for the complete satellite system engineering design including payload(s). FORMOSAT-5 will operate in a sun synchronous orbit at 720-km altitude with 98.28-degree inclination angle. The optical Remote Sensing Instrument (RSI) can provide 2-m resolution panchromatic and 4-m resolution multi-spectral images. A Cassegrain type of RSI with two reflective aspheric mirrors and spherical corrector lens is designed. The space thermal environment is one of the important factors that affect the image quality of a space optical RSI. Thermal deformation of RSI structure and mirrors will impact the optical performance. Some preliminary thermal and opto-mechanical analyses are conducted. The relationship among temperature distributions, thermal deformations and image degradation of on-orbit behaviors are discussed. The thermal induced instabilities are incorporated in optical models (e.g., OSLO) by use of Zernike polynomial coefficients to calculate the system MTF and WFE. Due to different thermal environments, on-orbit optical performances depend on imaging locations. Detailed studies and discussions are done in this paper to identify the major sources of thermal deformations and their impacts on image quality.