The impact of fog on activities such as land transportation, flight and marine operations, and military missions can be dangerous. Thus, this study developed a new and simple method for detecting fog areas over the Taiwan Strait more accurately during winter by using the Tropical Rainfall Measuring Mission Microwave Imager (TMI) Level-1B calibrated brightness temperature (TB) data, combined with ground station fog observations. This method is based on the microwave TB differences among four TMI channels, including horizontal and vertical polarization TBs of 37.0 and 85.5 GHz, for heavy fog and clear weather conditions. The method can be applied to large areas during the daytime and nighttime and distinguish fog from low clouds. It involves selecting fog cases and recognizing fog areas, removing relevant land data and calculating TMI TB statistics, establishing TB fog criteria, and finally validating the fog criteria through analysis of independent cases. The results indicated that the fog detection accuracy reached 75%. Among the four channels of the TMI, the 85.5 GHz horizontal polarization exhibited the highest fog sensitivity.