Ultrafine glass wool felt has excellent performance in heat insulation, sound absorption and filtration as well as A-level fire-retardant property, while high-temperature treatment has a great influence on its performance. In this paper, ultrafine glass wool felt is prepared by centrifugation and subsequently is heated in the temperature rang of 100—550 ℃. The microstructure, mechanical properties and thermal conductivity are characterized by metallurgical microscopy, universal materials tester and Netch thermal conductivity analyzer. The results show that, with the treatment temperature increasing, the diameter of the fiber and the mass loss increase while the pore diameter decreases, especially at 400 °C or higher temperature. The tensile strength decreases gradually, also obviously above 300 °C. The thermal conductivity increases gradually, which rises gently below 400 °C but accelerates greatly after that.