Heat exchanger is a key component in aircraft engine bleed air temperature regulation systems. The fouling of heat exchanger reduces the heat transfer efficiency and introduces additional resistance to flow, causing extra drag and fuel consumption. This paper proposes a heat exchanger fouling detection method based on the valve control command of the normal engine bleed air temperature regulation system in operation. With this method, no additional sensor measurement is required for the heat exchanger fouling detection. Computer simulations and experiments have been conducted, and the results have confirmed the effectiveness of the proposed method.