Based on perturbation theory, a new equation of state is developed for real electrolyte aqueous fluids, which is composed of ion-ion, ion-dipole, dipole-dipole, Lennard-Jones dispersion, hydrogen bonding association and hard sphere repulsion terms. The MSA equation from Blum is adopted instead of the perturbed electrostatic term, which simplifies the calculation. The SAFT equation is adopted for the calculation of hydrogen bond association for water molecules. For each electrolyte, only one adjustable parameter, i.e., the cation soft sphere diameter, is regressed. The experimental mean ionic activity coefficient data of 30 aqueous electrolytes (including 1:1, 2:1 and 1:2 electrolytes) were correlated. The parameters thus obtained can be used to predict the densities of these aqueous single electrolyte solutions and the mean ionic activity coefficients in mixed electrolyte aqueous solutions without any additional adjustable mixing parameters.