This study prepared a 5052H112 alloy under different cold-rolled reductions to explore its microstructure and mechanical properties. The experimental results indicated that the cold-rolled specimens had good tensile properties and better vibration fracture resistance, due to the high densities of dislocation and small aspect ratio of crystal grain. The stress-elongation curves of all specimens showed the serrated yielding. The high densities of dislocation and small aspect ratio of crystal grain introduced by cold-rolling could hold the mobile dislocations long enough to let Mg atoms form atmospheres around them. In addition, the crack propagation behavior of all specimens showed that slip bands can be observed in the vicinity of the main crack and be suppressed by increasing the cold-rolled reduction. Crack propagation showed that rolled specimens exhibit decreasing crack propagation rates with increased matrix strengthening (cold-rolling). Therefore, it can be concluded that a large number of dislocation tangles introduced by cold-rolling are effective in improving mechanical properties.