Abstract: Wind tunnel testing serves as a critical method for studying the performance of high-speed aircraft. Enhancing the measurement accuracy of infrared characteristics in wind tunnel tests can provide more reliable data support for predicting aircraft infrared signatures and designing stealth performance, embodying significant engineering value. However, in wind tunnel tests, the interference of background radiation inside the wind tunnel makes it difficult to accurately obtain the true infrared radiation (IR) of the aircraft, directly affecting the execution of wind tunnel tests and the use of test data. The CFD (Computational Fluid Dynamics) method verified by simulation accuracy was used to simulate the state of a conventional hypersonic blowdown wind tunnel with the incoming Mach number at the test section inlet in the range of 3-10. A numerical study was conducted on correction methods for IR measurement in the 3-5μm band at a lateral 90° direction for the aircraft and lateral jet, aiming at the interference of wind tunnel background radiation. A progressive measurement procedure based on background field separation was proposed. Through a stepwise experimental design involving cold-model interference calibration and hot-target measurements, the interference signals were stripped. The relationships between the intrinsic radiation of the aircraft surface and lateral jet and the total IR received by the detector were established respectively. The interference factors in IR measurement were identified. The correction formulas for measuring the intrinsic IR of the aircraft surface and lateral jet were derived. The results show that the influence of interference factors on the IR of lateral jet is relatively small, ranging between 1.6% and 2.8%. When the incoming flow Mach number in the wind tunnel test section is between 3 and 5, the influence of interference factors on the IR of the aircraft surface exceeds 10%. Especially at a Mach number of 3, the proportion of interference signals reaches 70.72%.