The invention provides a relaxation ferroelectric monocrystal pyroelectric infrared detector and a preparation method thereof. The detector comprises a base, a shell, a sensing element, an upper electrode, a lower electrode, an absorbing layer and a current mode circuit, wherein the base is provided with pins, and the shell provided with a window and the base are packaged to form a holding space; the sensing element is arranged in the holding space and is made from relaxation ferroelectric monocrystal, and the thickness of the sensing elemtn is below 20 [mu] m; an upper electrode and a lower electrode are respectively arranged on the upper surface and the lower surface of the flexible element, the absorbing layer covers the upper electrode of the flexible element, and the current mode circuit is connected with the flexible element. The detector provided by the invention is high in response ratio, low in noise and high in detection ratio.