Abstract: Di-N-m-acetylenephenylphthalimideether (DAIE) and cyanate ester resin (CE) were used separately to modify the poly(phenyl(phenylethynylsilyl)oxidephenylborane) terminated with phenylacetylene (PSOB). Two types of fiber reinforced composites (PSOB-PTPE-DAIE and PSOB-PTPES-CE) were prepared. The thermal stability, mechanical strength, dielectric and water resistance properties of the cured resins were investigated. It was indicated that both of the two modified resins exhibited hightemperature resistance with temperature of 5% mass loss up to 534 °C and the maximum char yield of 62.9% at 800 °C in N2 atmosphere. The flexural strength of the two types of composites was more than 250 MPa at room temperature and reached the maximum of 202 MPa at 240 °C. They exhibited well water resistance and their saturated water absorbing ratio reached the minimum of 1.4%. In different processing conditions, the dielectric constant and dielectric loss angle tangent of the composite showed little changes and their dielectric properties were excellent.