Polyvinylsilsesquioxane(PVSQ) was synthesized with hydrolysis-polycondensation method from vinyltrimethoxysiloxane (VTMS),and the curing mechanism, curing temperature and thermal stability of PVSQ were studied.The results show that PVSQ,which use ethanol as solvent and be curable at 150 ℃,can be used as the matrix of silicone-based ceramifiable composites for thermal protection applications.Combined with ceramifiable fillers (e.g.,TiB₂,mica,etc.),the in-situ ceramization process of PVSQ ceramifiable composites was studied by SEM, EDS and XRD.The results indicate that the weight loss of PVSQ composites is only 0.29wt% to 4.15wt% and the volumetric shrinkage is 5.20vol% to 15.08vol% when annealed at 800 to 1 200 ℃ for 30 min.The in-situ ceramization reaction take place between the ceramifiable fillers and the pyrolytic products of PVSQ at high temperatures,and generate a large amount of glassy phase for matrix bridging, which further leads to the formation of dimensionally-stable ceramic product.