Abstract: The water accumulation inside urban steel box girders is highly concealed and harmful, and traditional manual detection methods are difficult to implement. Infrared thermal imaging detection technology has fast detection speed and high efficiency, but the analysis methods and applicable conditions of detection images need to be studied. Based on this, an experiment on infrared thermal imaging detection of water accumulation on the bottom plate of steel box girder was carried out, and the influence of detection period, weather conditions, and water accumulation depth was analyzed. A quantitative feature contrast index was proposed to characterize water accumulation, and a threshold was set to determine the difficulty of identification. The temperature difference between the top and bottom plates of the steel box girder was used as an environmental suitability index for infrared thermal imaging detection, and the relationship curve between the temperature difference between the top and bottom plates and the feature contrast was fitted. The experimental results showed that infrared thermal imaging technology could effectively detect water accumulation on the bottom plate of steel box girders. The conditions for effectively detecting 80 mm (depth) water accumulation and 40 mm (depth) water accumulation were that the temperature difference between the top and bottom plates was greater than 4.7 ℃ and 7.6 ℃, respectively. The clearer the weather was, the deeper the water accumulation was, and the longer the detection window period was.