To develop novel multi-component molten salt systems more effectively, we developed a simulative technique using the CALPHAD (Calculation of Phase Diagram and Thermodynamics) method to estimate the ionic conductivity and the melting point. The validity of this new simulative technique was confirmed by comparing the simulated ionic conductivities and melting points of typical high-temperature molten salts, such as LiF–LiCl–LiBr, LiF–LiBr–KBr, LiCl–LiBr–KBr, and LiCl–LiBr–LiI, with those reported data in the literature or experimentally obtained. This simulative technique was used to develop new quaternary molten salt systems for use as electrolytes in high-temperature molten salt batteries (called thermal batteries). The targets of the ionic conductivity and the melting point were set at 2.0Scm−1 and higher at 500°C, and in the range of 350–430°C, respectively, to replace the LiCl–KCl system (1.85Scm−1 at 500°C) within the conventional design of the heat generation system for thermal batteries. Using the simulative method, six kinds of novel quaternary systems, LiF–LiCl–LiBr–MX (M=Na and K; X=F, Cl, and Br), which contain neither environmentally instable anions such as iodides nor expensive cations such as Rb+ and Cs+, were proposed. Experimental results showed that the LiF–LiCl–LiBr–0.10NaX (X=Cl and Br) and LiF–LiCl–LiBr–0.10KX (X=F, Cl, and Br) systems meet our targets of both the ionic conductivity and the melting point.