Abstract: To achieve the coordinated improvement of multiple functions such as lightweight, high strength and low thermal conductivity of building insulation mortar, SiO₂ aerogel was prepared by sol-gel method and two-step modification method using tetraethyl orthosilicate and methyltrimethoxysilane as co-precursors, and it was loaded into expanded perlite by vacuum adsorption method to obtain SiO₂ aerogel-expanded perlite composite aggregate. The hydrophobic properties, mechanical properties and microstructure were characterized by contact angle, water absorption, cylinder compressive strength and microstructure. The geopolymer thermal insulation mortar was prepared by adding fly ash and slag as cementitious materials, NaOH and Na₂SiO₃ as activators. The effects of different water-binder ratios and mass ratios of composite aggregate to cementitious materials on the properties of geopolymer thermal insulation mortar were studied. The results show that the contact angle of the composite aggregate is 148.2°, the water absorption is 10%, and the hydrophobicity is good. The cylinder compressive strength is 0.69 MPa, which has good mechanical properties. When the mass ratio of composite aggregate to cementitious material is 0.40, the 28 d compressive strength of geopolymer insulation mortar is 5.91~6.96 MPa, the thermal conductivity is 0.239~0.299 W/(m·K), and the dry density is 980~1085kg/m³. The prepared geopolymer thermal insulation mortar meets the strength requirements of A5.0 autoclaved aerated concrete block, and the thermal insulation performance is close to the requirements of B07 autoclaved aerated concrete block.