Polycarlosilane (PC)，the precursor of SiC filer，was preparod by heating polydimethysilane in the presenoe of polyborodiphenylsiloxne (python) as catalyst. The properties of polydimethylsilane，polycarbosilane and polyborodiphenylsiloxane have been investigated by IR spectra，chemical analyses，thermal analyses and pyrolysis gas chromatograph. The suitable reaction conditions were found tentatively. The results showed that the properties of the precursor depended on reaction tempesature，additive amount of polyborodiphenylsiloxane and reaction period of polycarbosilane synthetized. Silion carbide fiber was made by melt-spinning the polycarbosilane，making the precursor fiber infusible by heating in air to about 200℃ and finally heating under nitrogen to 1250℃.