Abstract: The hot compressive behaviors of cold rolling and semi-solid heat treatment AZ91 magnesium alloy was studied on Gleeble-3500 thermal simulation machine.The adopted deformation temperature is 300-450 ℃, and strain rate is 0.001-10 s^-1.The hot compressive constitutive model of the materia was established based on the analysis of stress-strain curves under different deformation condition and Zener-Hollomom calculation.The results show that the true stress-true strain curve of semi-solid AZ91 magnesium alloy is single peak dynamic recrystallization.The rheological stress decreases with the increase of deformation temperature.The stress peak moves coupling with the increase of deformation temperature in the direction of strain reduction.The constitutive equation of the compressive rheological stress of semi-solid AZ91 is \dot\varepsilon=2.01×10²⁴ sinh (0.2682σ)^7.605268exp (-248140/RT) The stress peak of semi-solid AZ91 billet is lower than that of the cast AZ91 magnesium alloy, except in the conditions of strain rate of 0.01 s^-1 and temperature of 300 ℃, 350 ℃, 400 ℃, or strain rate of 0.1s^-1 and temperatures of 300 ℃.