Abstract: The correlation distance is an important parameter in geotechnical reliability analysis based on random field theory. Based on the geological investigation data of Ningbo subway station, the influence of sampling spacing on the correlation distance of Ningbo soft clay layer is analyzed, and the principle of determining the correlation distance of Ningbo soft clay layer with the varied sampling spacing is presented. In addition, we explore the diversity in correlation distances of Ningbo soft clay layer obtained by different soil parameters and different calculation methods, and Wilcoxon rank sum test method is used to analyze the differences statistically. The results indicate that there is no statistical significance in correlation distances obtained by different methods and parameters. According to the geological investigation data of Ningbo metro line No.1, the vertical correlation distance of the Ningbo soft clay is calculated, and the vertical correlation distances of the mucky clay layer and the silt layer are 0.436 m and 0.352 m respectively.