Abstract: The reason why the relational expression beteen crack tip opening displacement (CTOD) and thickness (thickness effect expression), which was established by YAN Pengshuai et al., has relatively large errors in the application of bridge steels was analyzed, and then the expression was improved by construction method. The improved thickness effect expression was verified with test values of CTOD of bridge steel specimens (14MnNbq steel, weld of 14MnNbq steel joint, 16Mnq steel) with different thicknesses. The results show that the reason for relatively large errors by the original thickness effect expression was that the function form of the plane stress part was unreasonable. The expression was improved by replacing the unreasonable function with a probability density function. The relative errors between the fitted values by the improved expression and the test values of CTOD of the bridge steel samples with different thicknesses were relatively small. Therefore, the improved thickness effect expression is suitable for different bridge steels.