The method of non-contact temperature measurement based on an infrared thermal imager is a promising approach for realizing the real-time temperature monitoring in friction stir welding (FSW). However, due to the emissivity variation of aluminum alloys with temperature, there exists a significant error between the radiation temperature measured by the infrared thermal imager and the actual temperature of aluminum alloys. Current solutions primarily involve applying blackbody material on the surface of aluminum alloy plate, but this method is not practical in engineering scenarios. To address the problem, a novel approach to iteratively correct the radiation temperature using the fitted emissivity model is proposed. First, the correction formula of radiation temperature and the model of emissivity are proposed. Second, the radiation temperature of the aluminum alloy plate is obtained through the FSW experiment. Finally, the actual temperature of the aluminum alloy plate is determined based on the iterative correction algorithm. The experimental results demonstrate that the average temperature error of the method is 8.645 K, which verifies the validation of the method.