Abstract: The semi-solid compression tests at deformation temperatures of 500,520,540 ℃ and strain rates of 0.1, 1,5 s^-1 were conducted on as-cast Mg-6Zn alloy by Gleeble-3500 thermal simulator. The true stress-true strain curves were obtained. The semi-solid compression constitutive model was established based on the true stress and true strain test data and Arrhenius model, and was verified by tests. The results show that the semi-solid compression deformation of Mg-6Zn alloy was divided into three stages of quasi-elastic deformation, strain hardening deformation and rheo-viscoplastic deformation. The peak stress decreased with the increase of deformation temperature or the decrease of strain rate. The correlation between the calculated values by the constitutive model and the test results of true stress was very good with the correlation coefficient of 0.947 and the average relative error of 5.57%, which indicated that the constitutive model could accurately predict the semi-solid compression deformation behavior of the as-cast Mg-6Zn alloy.