The phase diagram of the Au-In-Sn system is of importance in developing materials of high performance in electronic packaging as well as in simulating the interfacial reaction between Sn-In and Au. By using the calculation of phase diagram (CALPHAD) method, phase equilibria in the Au-Sn binary system has been optimized based on experimental thermodynamic property and recently renewed phase boundaries. Combined with two binary systems, Au-In and In-Sn, in addition to the reported experimental isothermal section of the Au-In-Sn system, the Au-In-Sn ternary system has been modeled, and three isothermal sections at 300 K, 500 K, and 700 K and three vertical sections, Au-In 0.2 Sn 0.8 , Au-In 0.5 Sn 0.5 , and Au-In 0.7 Sn 0.3 , of the Au-In-Sn ternary system have been further calculated.