In this work, in ternary mixed-electrolyte system (NaClCdCl2H2O), the mean activity coefficients of NaCl were measured with the potential difference method of the cell: Na ion-selective electroed (ISE)|NaCl(m(1)), CdCl2(m(2))|Cl ion-selective electroed (ISE) at 298.15 K. Also, we chose the total ionic strength I range from 0.0100 to 1.0000 mol.kg(1) for different ionic strength fractions y(b) of CdCl2 with y(b) = (0.8, 0.6, 0.4, 0.2, 0). The experimental results gave that the mean activity coefficients of NaCl can be interpreted well by the Pitzer models, Na-ISE and Cl-ISE had a good Nernst response. Furthermore, the mixing Pitzer ion interaction parameters of theta(Na,Cd) and Psi(Na,Cd,Cl) were fitted from Pitzers equation, and the two parameters were applied to compute the values of the mean activity coefficients of CdCl2.