The mean activity coefficient of NaCl in NaCl-SrCl2-H2O was measured at 308 K by the electromotive force method. The total ion strength range of the mixed solution was 0.0100-1.0000 mol.kg(-1), and the ion intensity fraction of y(b) of SrCl2, i.e., y(b) = (0.8, 0.6, 0.4, 0.2, 0). A liquid-free electrochemical cell (Na-ISE vertical bar NaCl(m(1)) SrCl2(m(2))vertical bar Cl-ISE) was used to determine the electromotive force of a NaCl single salt and mixed solution. According to experimental data, the activity coefficient diagrams were drawn. The experimental results show that the measured activity coefficients are accurate and reliable, and the average activity coefficient of NaCl decreases with the increase of the ion strength of SrCl2. Using the measured average activity coefficient of NaCl, the parameters of the mixed ion action of the Pitzer equation theta(Na+center dot sr2+) and phi(Na+center dot Cl-center dot Sr2+) are fitted. Furthermore, the Pitzer equation is used to calculate the average activity coefficents of SrCl2, the water activities a(w), osmotic coefficients Phi, and Gibbs free energies G(E) of the mixed system.