Fiberglass porous ceramics were successfully prepared via a foam‐gelcasting process with fiberglass and glass particles. The effects of fiber content on the rheology of foaming slurries and the structure and mechanical properties of as‐prepared porous ceramics were investigated. The results showed that as the ratio of fiberglass to glass particles increased, the thixotropy of slurries decreased, which affected the foamability of slurries. When the ratio of fiberglass to particle was 75:25, the slurries exhibited excellent flowability and outstanding foamability, which was beneficial to optimize the structure of pores and improve the properties of the porous ceramics. The porosity, compressive strength, and thermal conductivity of porous ceramics with a content of 75 wt.% fiberglass treated at 750°C were 78.3%, 2.15 MPa, and .11 W/(m·K) (room temperature), respectively. Therefore, the prepared porous ceramics with a ratio of fiberglass to particle 75:25 were regarded as an ideal candidate for thermal insulation materials.