In order to reveal the hyperplasticity mechanism of electromagnetic forming (EMF), the plastic instability mechanisms and formation behaviors of 5052 aluminum alloy sheets in pulsed magnetic tension process were investigated by theoretical analysis and microanalysis. Results show that inertia force plays an important role in dynamic forming, which has the suppression effect on structural instability and thus improves the formability of sheet and spreads instability. The nature of dynamic formation is much similar with that of quasi-static formation and no special formation structures arise in dynamic process for 5052 aluminum alloy sheets. The formation mechanism of both processes is dislocation slip mechanism. For quasi-static formation, the dislocations show a uniform single-slip pattern, fracture combined with dislocation tangling and climbing. While for dynamic formation, dislocation system tends to more slips, large areas showing clear cross-slip structures. The dislocation bands are narrower and much denser than those shown in the quasi-static process, and a much more uniform dislocation configuration is also exhibited after pulsed magnetic loadings. The characteristics of multi-slips and uniform effect of dislocations under pulsed magnetic loading conditions will result in much higher plasticity and strength of materials.