In this paper, the liquid-free battery Na-ISE vertical bar NaBr (m(1)), Na2SO4 (m(2)) vertical bar Br-ISE was used to measure the average activity coefficient of the ternary system NaBr-Na2SO4-H2O at 308.15 K by using the cell potential method. The ionic strength in the mixed solution is 0.01-0.70 mol.kg(-1) with different ionic strength fractions of Na2SO4, that is, y(b) = (0, 0.2, 0.4, 0.6, and 0.8). According to the Nernst equation, we can obtain the standard electromotive force E-0 (263.52), electrode response slope (26.4), and linear correlation coefficient R-2 (0.9998). These data indicate that the electrode response of the electrode pair is very good. On this basis, we used the Nernst equation to calculate the average activity coefficient of NaBr at 308.15 K in the NaBr-Na2SO4-H2O system. Finally, we got the corresponding mixed ion interaction parameters theta(Br,SO4), psi(Na,Br,SO4) fitted by multilinear regression. In addition, the permeability coefficient, water activity, excess Gibbs free energy, and activity coefficient of Na2SO4 can also be calculated by using the Pitzer equation.