The water inside the steel box girder has strong concealment. The traditional way is through internal manual inspection. However, internal inspection could not be performed due to the low girder height of the small and medium-span steel box girders. In this paper, infrared thermography is used to monitor the temperature change of the bottom plate continuously. A water-accumulation condition of 80 mm is set up inside the steel box girder. In this paper, the concept of defect contrast is proposed and the ease of defect recognition is expressed by defect contrast. Upon analysis of the infrared images, a linear relationship was identified between the temperature difference of the bottom plate and the defect contrast. This relationship was quantified through linear fitting, resulting in a function expression between the two variables. Infrared thermal imaging offers a wide recognition range and fast sweeping speed, which makes it an ideal technical means for water detection inside steel box girders.