The invention discloses a displacement interface stability evaluating device and method. The displacement interface stability evaluating device comprises a simulated annulus shaft, a rotary support, an eccentric adjusting part, a controller and multiple measurement pairs. Whether a displacement interface is stable or not can be judged by continuously acquiring the flowing speeds of annulus core speed zones at different measuring points in real time and distinguishing different displacement fluids and displacement interfaces based on time difference and further only by calculating the flowing speed change of front and rear edges of the displacement interface lengths and the flowing speed change of the front edges of the displacement interfaces within a certain time period, and an experiment for evaluating influence laws of different factors to interface stability can be performed through the device.