The invention provides an infrared radiation film transmittance measuring method based on an infrared thermal imager. The method at least includes the steps: providing a radiating substrate covered with an infrared radiation film; respectively acquiring two groups of exitance data of the radiating substrate at different surface temperatures in the same indoor environment; calculating transmittance of the infrared radiation film by using the two groups of exitance data. Each group of exitance data at least includes total infrared radiation exitance, radiation exitance of the infrared radiation film, reflected exitance of an ambient environment for the surface of the infrared radiation film, and radiation exitance of the radiating substrate through the infrared radiation film. The emissivity of the infrared radiation film and the transmittance of radiation energy of the radiating substrate through the infrared radiation film can be measured, so that a better infrared radiation film can be designed in a targeted manner, the radiating efficiency of a power device is enhanced, and the junction temperature of a chip is reduced.