Abstract: Ethynyl-terminated hydrogenated hyperbranched polycarbosilane (EHBP) was prepared by Grignard coupling polymerization and reduction reaction, in which chloromethyltrichlorosilane(Cl₃SiCH₂Cl) and ethynylmagnesium bromide(HC≡CMgBr) were chosen as materials. The chemical structure, curing behavior and thermal properties of EHBP polymer were characterized by FT-IR, NMR(¹H-NMR、¹³C-NMR、²⁹Si-NMR), as well as DSC, rheological analysis and TG. The process of ceramization was analyzed by XRD, Raman spectroscopy, SEM and HR-TEM. Results showed that the thermal and thermal-oxidation stability of EHBP polymer were greatly improved after the introduction of Si-H and unsaturated ethynyl groups at the end of the chain. The temperature of 5% mass loss(T_d5) and residual mass at 1 000℃ were 578.8℃ and 82.3% in nitrogen atmosphere, and 549.6℃ and 69.2% in air atmosphere, respectively. With the increase of heating temperature, the pyrolysis of EHBP polymer became more and more sufficient. When the temperature reached 1 600℃, the crystallinity of the pyrolysis product was perfect, and the product contained not only α-SiC crystals but also β-SiC crystals which had high temperature stability. All SiC crystals with the size of 15~20 nm could be uniformly distributed in pyrolysis product.