The geopolymer-based lightweight porous material was prepared using slag micronized powder as main raw material, mix solution of sodium silicate and sodium hydroxide as alkaline activator, and aluminum powder as foaming agent. The effects of foaming agent, water-cement ratio and naphthalene water reducer on the pore structure and physical properties of material were systematically investigated. The results show that Al powder reacts rapidly with alkaline activator to generate H2, which promotes the foaming of geopolymer slurry to form porous material, and the dry density and compressive strength of material decrease rapidly with the increase of the Al powder content. However, if Al powder content exceeds 0.40% (mass fraction, the same below), the cell increases sharply, resulting in cell aggregation and a significant reduction in strength. Raising the water-cement ratio can reduce cell growth resistance and promote a rapid reduction in density. Nonetheless, when the water-cement ratio＞0.40, the slurry viscosity and activator concentration significantly decrease, setting time prolongs, pore size increases and structure deteriorates. Thus, the optimal water-cement ratio is 0.35. In addition, naphthalene water reducer is effective in regulating the pore structure of porous geopolymer. Adding only 0.4% of naphthalene water reducer could promote uniform pore size distribution, enhance pore wall integrity, and improve the compressive strength of specimens.