Polycarbosilane(PC-P), which is a useful precursor for Silicon Carbide fiber with high mechanical properties and low specific resistance, was synthesized using polydimethysilane(PDMS) and 3%～7%wt polyvinychloride(PVC). The synthesizing condition for PC-P was determined. The composition, structure and property of PC-P were studied by element analysis, IR, XPS and GPC. The reaction mechanism was preliminarily probed.