This study presented the preparation and property characterization of polysaccharide aerogel as a kind of thermal insulation and flame-retardant biomass material. For safe and sustainable thermal insulation applications in buildings, a polysaccharide aerogel with both low flammability and thermal conductivity is of great importance. Therefore, the konjac glucomannan (KGM) /sodium alginate (SA) aerogels prepared by the sol-gel and freeze-drying method were investigated in this study. KGM was used as the skeleton material of aerogel and SA was selected due to its inherent flame retardancy properties, which easily formed a dense carbon layer barrier to prevent the fire. Results showed that the pore characteristics were impacted by both total solid mass and KGM/SA ratios. The mechanical properties were significantly enhanced by the formation of hydrogen bonding interactions between KGM and SA. Finally, the KGM/SA aerogel with the optimized formula had a peak heat release of 33.12 W/g, mechanical properties of 161.5 kPa, density of 0.0348 g/cm3, porosity of 95.67±0.11%, and thermal conductivity of 0.0385 W/(mK). These results revealed the great application potential of KGM/SA aerogel to be used in building fire protection and thermal insulation and guided the formulae design of polysaccharide aerogels.