Flat mirrors are often used for designing many optical measurement systems. These mirrors can be generally deformed with respect to environmental conditions during measurements. The detailed theory of deformation of the thin flat mirror, which vibrates in the direction of the normal to its surface, is introduced in our work. It is also derived the relation for the dynamic wave aberration. On the basis of this relation, it is carried out the calculation of the Strehl definition of the deformed mirror. Obtained results can be used for analysis of the influence of mechanical vibrations on the accuracy of optical measurement systems in various practical applications.