Most minerals in the nature exist as solid solutions. Calculating the thermodynamic properties based on their crystal chemistry characteristics is the basis for the theoretical study of the formation of minerals. In this paper, a fictive ternary model for the simulation of thermodynamic properties of binary mineral solid solutions is introduced. The thermodynamic properties of diopside-jadeite solid solution were calculated. By constructing one ordered intermediate state mineral and considering both long-range order and short-range order effects, the model could calculate equilibrium thermodynamic parameters such as free energy, enthalpy, and entropy for a specific composition based on the rule that the lowest free energy corresponds to thermodynamical equilibrium mineral solid solution. The activity-composition relationship and temperature-composition phase diagram of diopside-jadeite solid solution system were calculated with omphacite as ordered intermediate state. It is found that the order/disorder transition of omphacite with increasing temperature is a first-order phase transition with a transition temperature of 1 148±25 K, which is in agreement with the experimental results. The thermodynamic parameters of the diopside-omphacite-jadeite system obtained in this paper can be applied to the study of the eclogitefacies metamorphism of rocks with the MORB bulk composition by means of pseudosection thermobarometry method.