Based on the Miedema model and the Toop model, the activities of Mg and Zn in the Mg-6Zn-1Mn alloy were calculated by a thermodynamic calculation model for predicting the precipitation behaviors of the intermetallic compounds in ternary metallic alloys. The relationship between the temperatures and the Gibbs free energy change during the precipitation reaction of the primary strengthening phase MgZn was deduced. According to the calculation, the precipitating temperature of MgZn phase is 580 K, in good agreement with the homogenizing temperature of this alloy and approved by our thermal analysis experimental results