The polycarbosilane (PCS)，the precursor of SiC fiber，is synthesized at 450℃ for 6h in an autoclave by thermal decomposition of polydimethylsilane (PDMS). The composition and structure of the PCS are characterized by softening point, element analysis, IR, molecular weight distribution, and NMR. Structure model of PCS is therefore proposed. IR and NMR show the presence of SiC₄and SiC₃H structure units containing Si-CH₃, Si-CH₂-Si, and Si-H groups. Elemental analysis provides an empirical formula of SiC_1.87 H_7.13O_0.03. Compared with the PCS with close softening point synthesized at normal pressure, the PCS synthesized in the autoclave has a approximate elemental composition, higher Si-H bond content, higher molecular weight, but lower ratio of SiC3H and SiC₄.