Abstract: Foamed geopolymer specimens with four distinct densities (400, 600, 800, and 1000 kg/m³) and two height-width ratios (1 and 0.5) were prepared for the quasi-static compressive test to investigate the influence of density and height-width ratio on its failure mode, mechanical and energy absorption properties. By introducing the material damage with the Weibull distribution function, a one-dimensional damage phenomenological constitutive model for foamed geopolymer, considering the factors of density and height-width ratio, was proposed based on the Avalle model. Furthermore, 13 groups of specimens with varying density gradients were fabricated for quasi-static compressive tests to investigate the potential for load transfer mitigation and multi-stage protection in the field of structural protection. The test results show that under the same height-width ratio, the total energy absorption of foamed geopolymer specimen increases with increasing its density. With increasing the specimen density from 400 kg/m³ to 1000 kg/m³, the total energy absorption increases by 272.94% with height-width ratio of 1 and 609.13% with height-width ratio of 0.5, respectively. At the same density, reducing the height-width ratio significantly improves the energy absorption performance of the specimens. With decreasing the height-width ratio from 1 to 0.5, the specific energy absorption of 800 kg/m³ specimens exhibit the maximum increase, which is 146.21%. The proposed one-dim-ensional damage phenomenological constitutive model could accurately predict the mechanical and energy absorption properties of foamed geopolymer, based on comparisons with test results. For foamed geopolymer with density gradients, a rational gradient configuration could delay or even prevent the formation of through-cracks, enabling progressive layer-by-layer crushing. This feature demonstrates promise in addressing the multi-stage protection requirements of energy-absorbing components.