In the present study, in order to predict the Water activity of aqueous electrolyte system, 21 cases have been regarded aqueous solution (AClx=1,2;A=Li,H,Na,K,NH4,Cs,Mg,Ca,Ba and Bj=1,2SO4; B=Li,Na,K,NH4,Mg,Mn,Ni,Cu,Zn and NaCl+LaCl3(aq) , NaCl+MgCl (aq) , Na2SO4+MgSO4(aq)). For this study, Extended UNIQUAC Thermodynamic model is desired, that adjustable parameters, are optimized with genetic algorithm. Results Extended UNIQUAC model optimization by genetic algorithms, has produced favorable results; so that the minimum and maximum total system error electrolyte is 0.00017 and 0.05, respectively. For electrolyte system, in this study, temperature 298.15, and the pressure of 1 bar is considered. The maximum concentration of 6 has been studied.