Based on the Miedema formation enthalpy model, thermodynamic properties of Al-Er, Si-Er and Al-Si binary alloy were calculated, and Er was chosen as the asymmetric component in asymmetric model. Combining with Toop model, the enthalpies of formation, excess entropies and excess free-energy of Al-Si-Er ternary alloy were calculated. The results show that the enthalpies of formation, excess entropies and excess free-energy of the ternary alloy are all negative in the whole content range, and their values change obviously in the zone where the content of Er is high or low. Through calculating partial molar excess Gibbs energy of three components (Al, Si, Er), isoactivity curves of all components of Al-Si-Er ternary alloy at 1073 K were obtained. The curves show that these values of all components of Al-Si-Er ternary alloy decreased dramatically along with the diminishing of molar fraction, and these activity values of Al, Si and Er were slight in the central zone of composition triangle of ternary system. It indicates that there was strong interaction among the three components, and they were easy to form ternary intermetalic compound, which coincided with the ternary phase diagram.