High-modulus carbon fiber reinforced plastic (CFRP) is one of the most widely used materials for manufacturing of precision dimensionally stable structures for the space telescope design. For instance, it is used for the design of the primary mirror panels of the Millimetron space observatory. To ensure unique characteristics of the primary mirror, extremely high requirements for the accuracy of the reflecting surface of the panels, as well as for their thermal stability in operational conditions at ultra-low temperatures (up to 4.5 K) are set. It is possible to meet these requirements due to the fact that the selected high-modulus CFRP is characterized by a combination of low coefficient of linear thermal expansion with high specific stiffness and low density. However, despite the unique physical, mechanical and thermal properties of the material, there are number of factors caused by both CFRP nature and technological features that can significantly decrease the accuracy of the reflecting surface and the thermal stability of the primary mirror panels. The variation in physical and mechanical properties of the ply is particularly distinguished among these factors. The paper presents a study of the effect of variation in physical and mechanical properties of the ply on the main parameters of the thermal stability of the primary mirror panels (root-mean square of the reflecting surface and the focal length) of the Millimetron space observatory carried out on the basis of numerical engineering analysis via the finite element method. The panel design is described, the methods of the development and verification of its mathematical model, the features of postprocessing the results of temperature deformations analysis are presented. The results of the numerical analysis allow to conclude that the primary mirror panels of the Millimetron space observatory have a high thermal stability and the requirements for the accuracy of the reflecting surface of the panels under operational conditions are met.