Red mud from Bayer process contains more hematite and has lower activity, which limit its resource utilization to some extent. The Bayer red mud, blast furnace slag and fly ash were mixed at a mass ratio of 5∶3∶2, sodium silicate and sodium hydroxide were used as alkali activator, calcium stearate was used as foam stabilizer and hydrogen peroxide was used as foaming agent, then the foamed geopolymer was prepared. The influence of alkali activator modulus on the bulk density, strength and microstructure of red mud-based foamed geopolymer was investigated. The results show that, with the increase of modulus of alkali activator, the bulk density of foamed geopolymer increases first and then decreases, and the compressive strength decreases gradually. When the modulus of alkali activator is 1.4, the bulk density at 28 d of foamed geopolymer is the lowest (195.86 kg/m3), the compressive strength at 28 d is the highest (0.55 MPa), and the specific strength reaches the highest (2 821.12 N·m/kg). The results show that the foamed geopolymer has the best performance of low density and high strength when the modulus of alkali activator is 1.4.