The invention discloses a viscous oil thermal recovery core displacement test device which comprises a core clamp, a cooling device, a high-pressure oil and water separator and a back-pressure valve. A backpressure end of the back-pressure valve is connected with a back-pressure automatic controller, two ends of the core clamp are connected with pressure difference measuring devices with three different measuring ranges, and the cooling device controls the temperature by the aid of constant-temperature water bath. A test pressure difference stabilizing method of the test device includes the steps: cooling high-temperature fluid generated by a core by the cooling device; enabling the cooled fluid to enter the high-pressure oil and water separator to finish on-line separation of oil and water, and metering output oil quantity; enabling separated water to control outlet pressure of a core through the back-pressure valve under the action of the back-pressure automatic controller; enabling water flowing out the back-pressure valve to enter a beaker, and weighing and counting weight by a balance. The test device can accurately controls backpressure, back-pressure stability is improved, so that pressure difference fluctuation at two ends of the core is reduced, and pressure difference changes of various stages in the core displacement process are accurately acquired.