To solve the transmittance measurement problem of large-aperture infrared optical systems in complete machines， a single-channel infrared transmittance measurement method based on grayscale images is proposed， and a transmittance measurement device is established. First， a blackbody and a target are placed at the focal plane of the infrared optical system under measurement according to the specified position relationship， and an infrared thermal imager is placed at the entrance port of the infrared optical system to capture the radiation image. Then， the infrared thermal imager is placed in front of the blackbody to capture the radiation image of the blackbody. Finally， the mean value of the radiation image and the mean value of the blackbody radiation image are calculated； the ratio of the two values is the transmittance of the measured system. Upon measuring the infrared transmittance of a large-aperture infrared optical system by using the proposed method， the results indicate that the method can accurately measure the transmittance of large-aperture infrared optical systems. The absolute error between the measured value and the design value is 0.85%. Analyzing the uncertainty of the test results reveals that the transmittance measurement accuracy of the proposed method is approximately 1.04%. This method entails a simple measurement process and is effective for evaluating the transmittance of large-aperture infrared optical systems in complete machines.