•Hydrogel-born foam is successfully fabricated by simple microwave-induced foaming.•The conversion of amide to imide can tune the hydrophilic/hydrophobicity of the foam.•The foam shows excellent mechanical, thermal insulation and fire-retarding properties. Hydrogel-born foam materials have been intensively developed because of great potential in various fields and environmental friendliness with water as foaming agent. However, the energy-consuming nature and pore collapse in the preparing and using process limit their application. Herein, a facile strategy is present to fabricate a dimensional stable hydrogel-born foam by simple acid-heat treatment and subsequent fast microwave foaming. The microwave heating rapidly evaporates and expands water into cells in the gel while the heat softening of the hydrogel further facilitates the formation of the cells. Moreover, the hydrophilicity/hydrophobicity and the crosslinking degree of the foam can be regulated by controllable conversion of amide groups of the hydrogel to the imide groups during the acid-heat treatment. As a result, it successfully avoids the pore collapse in both of the preparation and use process. With hierarchical pore structure and controllable moisture absorption, the foam shows with excellent dimensional and thermal insulation. In addition, the foam provides superior mechanical properties and fire retardance over traditional foams. The flame-retardant mechanism was proposed. The fabrication of this hydrogel-born foam is facile, flexible and can be adapted for extensive applications in building structures.