Poly(silphenylene-siloxane)s had been synthesized through dehydrocarbon polycondensation of 1,4-bis(dimethylsilyl)benzene (BDSB) and dialkoxysilane. The polymer composition and structure was characterized by 29Si-NMR. The influence of temperature, B(C6F5)3 concentration and monomer structure on the composition and the microstructure of the copolymers were investigated. It shows that elevating reaction temperature or using substrate (R’O)2R2R1Si with bulk organic group of R1 or R2 augments the run number value of the polymers, resulting in more alternating structure. Polymers with lower run number are obtained while increasing the B(C6F5)3 concentration. Using dimethyldimethoxysilane in stead of dimethyldiethoxysilane as monomer, the products are more alternating. Owing to the cyclization reaction of the side products of Si―H/Si―OMe exchange reaction, the mole ratio of diorganosiloxane unit and silphenylene unit in the copolymers is some 28/72－36/64, much lower than the mole ratio of the fed monomers, 50/50.