The thermal insulation material fabricated by sodium silicate has the advantages of lightweight and low thermal conductivity, however, its poor water resistance needs to be improved. In this paper, the thermal insulation materials were fabricated by an intermediate-temperature sintering method with boric acid as a modifier, and the effects of boric acid dosage on the microstructure and properties of the materials were investigated. The results show that boric acid can adjust the frame structure of Si-O tetrahedron and effectively reduce hydroxyl groups in the thermal insulation material. In addition, it improves the softening coefficient of the thermal insulation material, and inhibits the leaching of silicate ions and sodium ions in the solution, which improves the water resistance of the material. When boric acid content is 1.00% (mass fraction), the softening coefficient increases from 0.519 to 0.701, increasing by 35%. At the same time, boric acid can make the pore size distribution of the thermal insulation material more uniform, and improve its compressive strength. But the thermal conductivity and apparent density of the thermal insulation material will increase slightly. The thermal conductivity, apparent density and compressive strength of the thermal insulation material modified by 0.75% boric acid are 0.052 W/(m·K), 128 kg/m3 and 0.442 MPa, respectively. And it satisfies the performance requirements of inorganic thermal insulation materials.