This work deals with finding the critical condition of capillary wave instability development in a thin charged liquid film on the surface of a hard core in an external electric field, taking into account the disjoining pressure. It is shown that the critical values of the Rayleigh and Taylor parameters for instability development increase sharply with decreasing thickness of a liquid film. This testifies to the stabilizing influence of the disjoining pressure. The influence of viscous dissipation of capillary wave energy on the critical condition for their instability development is evaluated. The analytical expression is obtained which connects the critical values of the system charge and of the external electric field intensity for instability development with those of the thickness of a liquid film and dimensions of a hard core, as well as with those of the coefficients of surface tension and the kinematic viscosity of liquid.