A soft polyurethane elastomer containing isocyanurate ring prepared by reacting polyoxypropylene polyol having an OH equivalent molecular weight of about 500-3,000, and a functionality of about 2-5, and polyisocyanate compound in the presence of tertiary amine catalyst, and if desired further combined with organic tin catalyst. This process is advantageous in facilitation of material control during curing reaction, low exothermicity and high thermal stability of the resulting polymer. Further the properties of the polymer are improved by using ethyleneoxide addition polyoxypropylene polyol instead of the above polymer and a specific tertiary amine as a catalyst. The elastomer thus prepared is significantly suitable for tire filling material.