The invention provides a pH-responsive modified polyacrylamide oil displacement agent and a preparation method thereof. The oil displacement agent is composed of 10-85wt% of acrylamide, 10-40wt% of sodium acrylate, 5-35wt% of poly N, N-diethylamido ethyl methacrylate, 5-15wt% of acrylic polyethylene oxide-propylene oxide, auxiliary, initiator and water. The invention further provides a preparationmethod of the oil displacement agent. The oil displacement agent can effectively lower viscosity of crude oil during crude oil extraction, is highly responsive to acidity and alkalinity and can effectively increase crude oil recovery ratio.