Constructing three-dimensional (3D) porous structures is an effective method to improve microwave absorbing performance due to the multiple reflections. However, in traditional 3D porous structures, multiple reflections only occur within the pores. Promoting multiple reflections beyond the pores should further improve the absorbing performance. Herein, a 3D porous composite foam with sandwiched cell walls was developed to boost multiple reflections in both the submillimeter-scale pores and the submicron-scale cell walls, achieving multiscale microwave absorption. Polydopamine (PDA)-modified polyimide foam (PIF) was used as the 3D porous skeleton. The cell walls of the PDA@PIF were sandwiched by Ti 3 C 2 T x MXene. The resulting Ti 3 C 2 T x @PDA@PIF can absorb over 90% of the incident microwave over the whole X band, which is contributed by multiscale multiple reflections, conduction loss, and interfacial polarization. Meanwhile, the composite foam exhibits excellent flexibility and a low density of ∼30 mg cm −3 . This work offers a realistic approach for lightweight, flexible, and broadband microwave absorbers, enriching the structures for effective electromagnetic protection.