Abstract: Trifunctional polysiloxane flame retardants were synthesized through hydrolysis condensation reaction. The influence of the Karstedt catalyst (KC) on the polysiloxane/PC composite and the flame retarding mechanism were studied using limiting oxygen index (LOI) test, cone calorimeter, TGA, PY-GC/MS and SEM. The results show that the KC promotes the crosslinking of the trifunctional polysiloxane containing vinyl groups and increases the thermal stability of the polysiloxane/PC composite. Furthermore, KC restrains the release of the combustion heat of the polysiloxane/PC composite, which delays the emission of smoke, CO and CO₂ and reduces the peak of the emission of smoke. Thus the flame retardancy of the polysiloxane/PC composite is enhanced and the danger for combustion is reduced. However, the effect of KC on the LOI of the polysiloxane/PC composite is quite complicated. The LOI of the polysiloxane/PC composite with KC depends not only on the amount of the char, but also on the structure of the char layer. KC could promote the formation of more compact and uniform char layer of the polysiloxane/PC composite during the combustion, thus the LOI of the composite is increased.