In a process for preparing a smoke-retardant polyisocyanurate foam by reacting an organic polyisocyanate in the presence of a blowing agent and an isocyanate-trimerization catalyst, an improvement is characterized in that the reaction is conducted in the presence of an organosilicate having the formula Si(OR).sub.4 or R.sub.n Si(OR').sub.4-n wherein R and R' are the same or different and represent aliphatic, alicyclic and aromatic hydrocarbon groups having 1-12 carbon atoms or substituted derivatives thereof and n is an integer of 1 to 3.