This paper takes the typical AZ31B magnesium alloy, which is increasingly widely used in engineering, as the research object and aims to study the fracture properties of magnesium alloys and their damage mechanisms under different loading strain rates and loading modes. EBSD, trace method, and other techniques are applied to study the fracture modes at a low strain amplitude of 0.42% and high strain amplitude of 1.746%, to explore and compare the type and number of microcracks at these two strain amplitudes, and to analyze the reasons for the formation of different types of microcracks. The results show that the experimental 0pass and 4pass start to fracture when they are numerically simulated impact loaded at impact velocities of 225m/s and 260m/s, and the larger the value of impact loading, the more complete the fracture occurs. The fatigue fracture occurred at TBs, GBs, and PSBs with a strain amplitude of 1.746% and 0.42%, respectively. The SF of PSBs was mainly distributed in the range of 0.8-1.0, while that of TBs was most distributed in the interval of 0-0.2 and 0.6-0.9. The larger the SF, the more prone to sprouting of microcracks. The smaller the value of m′, the larger the value of Φ, the more likely to promote microcracks sprouting at the grain boundaries.