The formation mechanism of bi-modal foams containing cells of a relatively large size and those of a small size prepared by a two-step depressurization approach was studied through in situ visualization. The results demonstrate that the cells produced by the first depressurization do not coalesce into larger cells but shrink spontaneously in the pressure holding stage. In the second depressurization, a large number of new cells nucleate and grow into the smaller cells, driven by diffusion; meanwhile, after the holding stage, the remaining cells again expand and evolve into the final larger cells, driven by hydrodynamic forces. Based on the above mechanism, increasing the dense of large cells can be achieved by increasing the depressurization degree or reducing the holding time while an introduction of inorganic particle or an enhancement of intermediate pressure is propitious to form denser small cells.