Abstract: This paper reports the high temperature tensile deformation behaviors and mechanisms of twin-roll cast AZ31 magnesium alloy. At constant temperatures ranging from 300℃ to 450℃, the elongation-to-failure test was conducted under constant tensile strain rates of 10^-3 s^-1 and 10^-2 s^-1, and the strain-rate-change tensile test was performed under strain rates from 2×10^-4 s^-1 to 2×10^-2 s^-1. The elongations of specimens at 400℃ and 450℃ are both higher than 100% when the tensile rate is 10^-2 s^-1. But when the tensile rate is 10^-3 s^-1,the elongation values at 400℃ and 450℃ are both higher than 200%, the stress exponent n is about 3, the creep activation energy Q is 148.77 kJ·mol^-1, and the deformation mechanism is a collaborative mechanism of grain boundary sliding and solute-drag dislocation creep. Optical microscopy and scanning electron microscopy observations show that coarse grains develop at the failure ends due to dynamic recrystallization and grain growth, and the fracture is of a ductile pattern caused by cavity growth and interlinkage.