The embodiment of the invention discloses a system and a method for determining settling stability of a working fluid, the system includes a sample cup, a heating sleeve, a density test component, a connecting cover and a pressure pipe joint, the sample cup is arranged in the heating sleeve, the connecting cover is arranged at the opening of the upper end of the sample cup, a vibration element and a vibration sensor are connected, the vibration element passes through the connecting cover and extends into the sample cup; the pressure pipe joint passes through the connecting cover, one end of the pressure pipe joint is connected with the sample cup, and the other end of the pressure pipe joint is connected with a gas cylinder. Through the system, the working fluid can be heated to preset temperature under high pressure environment, the density of the working fluid at the preset temperature is tested, and the settling stability of the working fluid is determined according to the change of the density. The method solves the technical problems that the settling stability of the working fluid under the high pressure environment cannot be accurately evaluated quantitatively by the method in the prior art.