Disclosed are a chemically modified polyolefin elastomer composition that is highly compatible with thermoplastics and useful in improving the low temperature impact strength of thermoplastics, and a method for preparing the chemically modified polyolefin elastomer composition. The elastomer composition includes a polyolefin resin and a chemically modified ethylene-α-olefin elastomer mixed with the polyolefin resin. The chemically modified ethylene-α-olefin elastomer includes an ethylene-α-olefin elastomer backbone and grafted branches from the backbone. The branches includes polymers of one or more monomers comprising an unsaturated organic compound containing at least one carbonyl group. The method of producing the composition includes mixing an ethylene-α-olefin elastomer and a polyolefin resin to provide a mixture thereof, to which added is one or more monomers having an unsaturated organic compound containing at least one carbonyl group. Then the one or more monomers are polymerized in the presence of a grafting initiator.