Fatigue crack is a major damage form of orthotropic steel bridge decks. Due to the shielding effect caused by coating, fatigue cracks are difficult to detect. Furthermore, the darkness inside steel box girders makes manual visual inspection inefficient and prone to the high rate of missed detections. In order to explore a rapid and efficient method to detect fatigue crack, this study employs infrared thermography to detect cracks in orthotropic steel bridge deck. Firstly, the temperature transmission blockage effect caused by fatigue crack was analyzed, and two fatigue crack identification indicators were proposed. Secondly, the thresholds for identification indicators were established through comparing continuous infrared image series. Finally, optimal detection time for fatigue crack was determined based on these thresholds. The experimental results show that there is a temperature jump in the fatigue crack area, which is obvious comparing to intact area. Temperature jump value and expected offset rate of temperature derivative can effectively characterize the temperature transmission blockage effect caused by fatigue crack. According to the identification limitation for infrared image through visual, the threshold of temperature jump value and expected offset rate are set to 0.75 ℃, 45% respectively. The higher the identification indicator is, the more obvious the fatigue crack is in infrared image. The period during 10:30 to 16:30 is appropriate to get infrared image for detecting fatigue crack because the identification indicators are higher than thresholds.