The precipitation mechanism of secondary-precipitated phase β in AZ80 magnesium alloy during the hot compression deformation under different temperature was investigated. The results show that there are β-Mg17Al12 phases with different morphology and precipitation mechanism in the microstructure of AZ80 alloy deformed both in two-phase region、critical region and single-phase region. β-Mg17Al12 phases were directly deformed and broken to be strip form during the compression under 200、250、300 and 350°C. A lot of fine granular second phase particles precipitated with the grain refinement simultaneously in AZ80 alloy deformed in two-phase region with tremendous deformation. There are also fine granular second phase particles precipitated in the alloy deformed in critical region. There are massive β-Mg17Al12 phases precipitated in the alloy deformed under higher temperature such as 350 and 400°C during the water cooling after the compression, the high power observation of which is fine and tightly lamellar microstructure. Compared with coarse reticular β-Mg17Al12 phases in the as-cast microstructure of AZ80 alloy, the morphology of β-Mg17Al12 phases precipitated during the hot compression has obvious improvement.