The invention provides a preparation method of polymer polyol and the polymer polyol prepared by using the method. The preparation method comprises the following steps that a mixture of an olefinic unsaturated monomer, basic polyether polyol, a stabilizer and a chain transfer agent is reacted at a certain temperature and pressure in the presence of a radical initiator, then a shell prepolymer is added, ageing is conducted at a certain temperature, the unreacted monomer and chain transfer agent are removed in the aftertreatment process, and the polymer polyol is obtained. Through the method, when a low-VOC product is prepared, product discoloration caused by long-time heating can be avoided, and the yellowing phenomena of down-stream foam products are reduced.