The invention discloses an oilfield produced fluid heat energy recovery device and method, and belongs to the technical field of heat energy recovery. In the device, a volume-variable separation tank comprises a tank body and a piston arranged in the tank body, wherein the piston can axially move in the tank body and divide the tank body into an upper cavity and a lower cavity; a check valve is arranged in the middle of the tank body, a first control valve is arranged at the upper portion of the piston, and a second control valve is arranged at the lower portion of the tank body. The gas phase outlet of a steam separation tank, a pressurizing pump, the check valve and the feed port in the middle of the volume-variable separation tank are sequentially connected; the outlet of the lower portion of the volume-variable separation tank, the second control valve, a third control valve and one inlet of an ejector are sequentially connected; the first control valve, the outlet of the upper portion of the volume-variable separation tank, the associated gas inlet of a heat exchanger and the associated gas outlet of the heat exchanger, the fuel inlet of a steam boiler, a fourth control valve, the other inlet of the ejector, the outlet of the ejector and a thick oil heater are sequentially connected; and the boiler water feed outlet of the heat exchanger is connected with the boiler water feed inlet of the steam boiler.