The polycarbosilane (PCS), the precursor of SiC fiber, was synthesized at the high temperature and under high pressure by thermal decomposition of liquid polysilane (LPS) which was obtained by thermal decomposition of polydimethylsilane (PDMS). The influence of synthesis conditions on final pressure at the room temperature, Si-H bond content, PCS yield, softening point, the molecular weight and distributions, and spinnability were investigated. The results showed that the molecular weight and softening point of PCS increased while the reaction temperature increased. The spinnability of PCS, however, decreased gradually. Suitable reaction time for the synthesis of PCS from LPS in autoclave should be 3 to 4 hours at 460℃, or 6 to 7 hours at 450℃. The as-synthesized PCS, in which the high molecular weight content was about 5～10wt% with softening point about 210 to 230℃, Si-H bond content more than 0.9, and good spinnability, was the suitable precursor for the preparation of SiC fiber.