Taking the assumption that activity of silver ion and sulfate ion of very low concentration is constant in any aqueous solution,the primary cell was designed for the cell reaction of Ag_{2}SO_{4}(s)→2Ag^{+}(aq)+SO_{4}^{2-}(aq).The compensation method was used to obtain the electromotive force of the primary cell when the concentrations of AgNO_{3} and K_{2}SO_{4} changed.The relationship between electromotive force and the mean activity coefficients of electrolytes as well as the solution concentration was deduced according to the Nernst equation and the limit Debye-Huckel formula.Standard electromotive force E ^{⊖} was obtained by extrapolation.Finally,the solubility product of silver sulfate was calculated through the relationship between equilibrium constant and electromotive force of the primary cell.