Superplastic tensile tests of 7B04 Al-alloy plates with average grain sizes of 10 and 20 μm were carried out at strain rate of 3×10^-4 s^-1 at 530℃ and with various desired deformation degrees. The results show that, as the deformation degree increases, the evolution of cavity morphology of the alloys follows the following order: cavity nucleation → spherical cavity dispersion → nonspherical cavity elongation along the stretching direction → cavity coalescence along the stretching direction → large-size cavity coalescence in the non-stretching direction. In the deformation stage before tensile fracture, there were polymeric cavities larger than 260 μm in size. At the initial stage of coalescence, the cavities aggregate along the tensile direction did not lead to fracture immediately. Large-size cavities coalesce along the non-tensile direction, which is the basis for judging the instability of materials. According to the experimental data, the cavity growth equation was established and the Cavity Growth Mechanism Map (CGMM) was plotted, including equations related with the nucleation, diffusion growth, plastic growth and aggregation growth of cavities, based on the CGMM the cavity morphology and material instability can be judged. According to the evolution of microstructure a physical model of cavity diffusion and plastic growth was established, based on which the energy dissipation required by cavity evolution during superplastic deformation can be calculated and the energy dissipation diagram can be drawn.