Determination of strains in structures made of composite materials has an important value for the development of theoretical models, creation of new prospective structures, and industrial inspection of the stability of shape of prototypes. The investigations should be performed using integral, noncontact, and sensitive methods. Various methods of measurements are discussed in the literature, at that, most of them are holography and speckle interferometry. However, when applying these methods directly, for example, when using holographic interferometry for measurements of thermal deformations of large-scale (larger then 1 m) objects, some problems can appear; these problems are caused by localization of the zero, necessity to increase vibroprotection of the objects to be studied, relatively high systematic error produced by thermal expansion of the interferometer elements, etc. At that, the methods of solution of such problems, which were successfully applied in classical schemes of interferometers, currently seem to be inefficient. We propose a new method of real-time differential holographical interferometry based on passive modulation of reference beam phase. The results of studies of deformations of carbon-plastic parabolic reflector of with 1 m maximum diameter of aperture subjected to uniform thermal loading, which simulates operation conditions, are presented. Using the results of experiments, recommendations on the design of joining elements of unfastening device of the reflector are made.