The invention discloses a polyurethane elastomer for automobile glass encapsulation and a preparation method thereof. The elastomer is prepared from isocyanate component A and polyhydric alcohols component B with a reaction injection moulding process, wherein the density is larger than 1000kg/cubic meter, and the index number of the isocyanate is 90 to 120, the component A is NCO-capped prepolymer with NCO % being 18 to 28 %, and is the reaction product of aromatic diisocyanate and polyhydric alcohols with low molecular weight; the component B comprises at least 60 percent of polyether polyol with the molecular weight of 1000 to 10000, 1 to 20 percent of arylamine, 0 to 20 percent of alcohol, and 0.2 to 0.5 percent of composite catalyst, and also comprises an antioxidant, a light stabilizer and color paste. The polyurethane elastomer of the invention has excellent mechanical capacity and moderate aging resistant performance, and is applicable to the glass encapsulation with various specifications, especially to the large, ultrathin or complex product.