In this paper, the cell potential method is used to determine the average activity coefficients of the NaBr–Na2SO4–H2O ternary system at 318.15 K. The ion-selective electrode and electrolyte solution are used to form a liquid-free battery. The ionic strength in the mixed solution is in the range 0.01–1.0 mol·kg–1 with different ionic strength fractions of Na2SO4, that is, y b = (0, 0.2, 0.4, 0.6, and 0.8). First, in a single salt solution, the response curve of Nernst can be obtained according to the Nernst equation. The standard cell potential E 0 = 251.03, electrode response slope k = 27.124, and linear correlation coefficient R 2 = 0.9998 are obtained in this work. In the same way, the average activity coefficients of NaBr in mixed electrolyte solution at 318.15 K were also determined by using the Nernst equation. Finally, the mixed ion interaction parameters θBr, SO4 and φNa, Br, SO4 of Pitzer equations can be fitted by the multilinear regression method combined with the average activity coefficients of mixed salt solution. In addition, the osmotic coefficients, water activity, excess Gibbs free energy, and activity coefficient of Na2SO4 can also be calculated using Pitzer equations based on the fitted θBr, SO4 and φNa, Br, SO4.