Abstract: Like most other soil layers, spatial variability of soil indexes also exists in the clayey layers. Such characteristic mainly results from geologic processes like deposition, diagenesis, et al. Determination of the scale of fluctuation is the key to know spatial variability of soil indexes. Therefore, this paper aims to analyze spatial variability of indexes of clayey soil in Nanjiang Port in Tianjin. It summarizes the spatial average method and its improved method at first. Combining geology data, the improved spatial average method is employed to analyze the scale of fluctuation of a typical soil layer in Tianjin Port in the vertical direction. The regional representative values are obtained, which are of high value on risk assessment in geotechnical engineering and determination of exploratory engineering intervals. In addition, the Bayesian inference is also discussed to revise the scale of fluctuation. Its results indicate that the Bayesian inference is much more reasonable. Furthermore, the scale of fluctuation of the mud and silt clay layer can be guidance for selection of sampling locations, and also valuable to reliability analysis of geotechnical engineering.