The invention relates to polymer polyol and a composition thereof and an application. The polymer polyol is a copolymer which is obtained through copolymerization of acrylic ester containing hydroxyl and acrylic ester free of the hydroxyl. The composition comprises matrix resin, the polymer polyol and a photochromic compound; and the solid content of the hydroxyl in the composition is 1.0-6.0%. The composition is a photochromic coating compound (or named a photochromic coating) which is good in weather resistance and good in compatibility with other functional coatings (such as a prganosilane hard coating or/and an anti-reflective coating). The composition is suitable for preparation of a photochromic coating product for pphthalmic use.