Soluble poly(dimethyl-co-methylphenylsilylene) (PDMPS) block copolymers with high molecular were prepared by copolymerizing dimethyldichlorosilane with methylphenyldichlorosilane using Na/K alloy and 2-N, N-dimethylaminonaphthalene co-catalyst, and the chemical structure was identified by IR, UV, and NMR. According to the mole ratio of the reactants and the reaction conditions, block units of (Me2Si) and (PhMeSi) in PDMPS copolymer, melting point, molecular weight change, yield, and solubility of PDMPS were measured. The block copolymer was white solid. The activation energies for the depolymerization of polydimethylsilane(PDMS), polymethylphenylsilane(PMPS), and PDMPS were determined by dynamic TGA data. The depolymerization activation energies for PDMS, PMPS, and PDMPS were 52, 71, 58 kcal/g$.$mole, respectively.