Polycarbonsilane (PCS) synthesized from polydimethelsilane (PDMS) is an excellent precursor to ceramic materials. On the base of an indirect way involving intercurrent liquid polysilane (LPS), this paper studied a direct way to synthesis PCS from solid PDMS. By the application of orthogonal design, influence of synthesis conditions on the structure and molecular weight of PCS were studied. And the optimized synthesis condition was obtained as reaction temperature, 455℃, pyrolysis temperature, 540℃ and retaining time, 6 hours.