The 7075 aluminum alloy is forged at different strain in different ways. OM(optical microscope) and TEM(transmission electric microscope) were employed to observe the microstructure of 7075 aluminum alloy after hot deformation. Experiment results indicated that 7075 aluminum can occur dynamic recrystallization under hot forging and fine grain can be obtained at the same time, whose recrystallization mechanism and nucleation mechanism are discontinuous dynamic recrystallization and subgrain rotation and polymerization respectively. Its critical strain value is related to deformation pass, the more passes are, the smaller critical strain value will become. Recrystallized grain size decreases with increase of strain, and dispersed second-phase particles play a very important role during dynamic recrystallization.