Abstract: A reaction system for preparing polymer polyol and a method for preparing polymer polyol. The reaction system comprises a reactor, a first circulation unit, a second circulation unit, and a flow direction switching unit, wherein the reaction cavity of the reactor is divided into a first reaction chamber and a second reaction chamber by a partition plate, and the top of the partition plate is provided with an overflow port to communicate the first reaction chamber and the second reaction chamber with each other; the first circulation unit enables the material in the first reaction chamber to circulate between the discharge port of the first reaction chamber and the feed port of the first reaction chamber; the second circulation unit enables the material in the second reaction chamber to circulate between the discharge port of the second reaction chamber and the feed port of the second reaction chamber.