Abstract: The 2.5D quartz fiber reinforced boron phenolic resin ceramizable composite was prepared by prepreg molding process. TG test, muffle furnace static ablation test, mechanical test, oxygen-acetylene test, field emission scanning electron microscope and other methods were used to analyze the thermal properties, mechanical properties and ablation resistance of 2.5D quartz fiber reinforced boron phenolic resin ceramizable composites. The test results show that the ceramic filler has a significant improvement on the thermal stability of the composites, and the temperature at the first stage of weight loss and the residual carbon ratio are significantly increased. After the muffle furnace pyrolysis, the 2D structure sample has many defects and damage between layers, and the bending strength is 14-20 MPa, the 2.5D structure sample maintains a good overall structure after pyrolysis, and the bending strength is 16-24 MPa. After the ablation of the 2.5D structure sample by the oxyacetylene ablation test, the mass ablation rate is 0.0413 g/s and the line ablation rate is 0.032 mm/s. After 60 s of ablation, the mass ablation rate is 0.0218 g/s, and the line ablation rate is 0.023 mm/s. The test results are better than the 2D structure samples. Through SEM and EDS analysis, the 2.5D structure samples have excellent thermal stress delamination resistance, and the fiber surface structure has stability to multiphase ceramics, which are the main reasons for low ablation rate of the composite.