Recently, the widely used Stokes parameters are generalized from one-point quantities into two-point counterparts, and used to determine the degree of polarization and degree of coherence of stochastic electromagnetic beams. On the basis of the generalized diffraction integral formula for an ABCD optical system in spatial domain, we derive a propagation law for the generalized Stokes parameters of a stochastic electromagnetic beam passing through an ABCD optical system, and use the Stokes parameters to investigate properties of the state of polarization of stochastic electromagnetic beams. As an example, we study the changes in the state of polarization of a stochastic electromagnetic Gaussian Schell-model beam propagating through a dual-focus system.