As an example of the La-Mg-Y system, the method how to set up the thermodynamic model of individual phases was introduced in the process of thermodynamic optimization. The solution phases (liquid, body-centered cubic,face-centered cubic, hexagonal close-packed and double hexagonal close-packed) were modeled with the Redlich-Kister equation. The compound energy model has been used to describe the thermodynamic functions of the intermetallic compounds in the La-Mg-Y systems. The compounds Mg2Y, Mg24Y5, Mg12La, Mg17La2, Mg41La5, Mg3La and Mg2La in the La-Mg-Y system were treated as the formulae (Mg, Y)2(La,Mg, Y), Mg24(La,Mg, Y)4Y, Mg12(La, Y), Mg17(La, Y)2,Mg41(La,Y)5, Mg3(La,Mg,Y) and Mg2(La,Y), respectively. A model (La,Mg,Y)0.5(La,Mg,Y)0.5 was applied to describe the compound MgM formed by MgLa and MgY in order to cope with the order-disorder transition between body-centered cubic solution (A2) and MgM with CsCl-type structure (B2) in the La-Mg-Y system. The Gibbs energies of individual phases were optimized in the La-Mg, La-Y and La-Mg-Y systems by CALPHAD technique. The projection of the liquidus surfaces for the La-Mg-Y system was predicted. The Mg-based alloys database including 36 binary and 15 ternary systems formed by Mg, Al, Cu, Ni, Mn, Zn and rare earth elements was set up in SGTE standard.