We present a theoretical study for the magnetic behavior of disordered Fe-Al alloys on the basis of a simple site-diluted Ising spin model. With the assumption that the exchange interaction $J$ depends on the Al concentration $q$, we calculate the thermodynamical properties of the model through a variational method for the free energy, based on Bogoliubov's inequality. An acceptable fit to the experimental phase diagram for Al concentrations in the range $0\ensuremath{\le}q\ensuremath{\le}0.475$ is obtained. A relation between the reduced magnetic hyperfine field and the theoretical magnetization is proposed, and rather good agreement with the experimental results at room temperature is also achieved.