An equation is derived for the effective radius of a femtosecond laser beam at its nonstationary self-action. For the single filamentation regime, the beam evolution has been qualitatively analyzed, and a model of beam passage through the global nonlinear focus is constructed. The model is based on the results of our numerical investigation. Filamentation of a femtosecond laser beam is considered as a process of formation of many local focuses along the propagation path. The dependence of the laser beam propagation coefficient (averaged effective beam divergence normalized to its initial value) on the initial beam parameters and the light energy absorbed in a medium is determined.