Getting activity of component is an important aspect of researching the thermodynamic characters of alloy. Under each model of predicting the thermodynamics characters of alloy, Miedema model has been developing as comparatively a good and accurate model in recent years. In this work, the formation enthalpy of Mg - Zn alloy has been calculated, according to the Miedema model. Further, the activity curve of Zn in Mg - Zn alloy at 1 000 K is calculated. The calculating result is that, as the formation enthalpy of Mg - Zn alloy is small, and in the procedure of calculating activity, excess entropy attributes a lot to the result, after considering excess entropy, calculating results and experimental values match each other very well.