Abstract: The edge effect, resulting from possible leakage in RCM test, and causing the chloride diffusion depth in the edge part of concrete specimen greater than that in the central part, was investigated systematically to analyze its influence on the chloride diffusion coefficient of concrete based on statistical analysis of the RCM test data of the concrete specimen, while the revised calculation expression of the diffusion coefficient was developed in this study. Firstly, variable voltage RCM tests were performed on 74 sets of concrete specimens. Secondly, the normalized depth was defined for chloride diffusion at observation points along the axial section of the specimen. Then the influence of edge effect on the diffusion depth and diffusion coefficient of chloride ion was quantitatively analyzed based on the results of variable voltage RCM test. Finally, the correlation between the diffusion coefficient for the center part of axial section of the specimen and that for the whole section were quantitatively investigated, thus providing a basis to calibrate the chloride diffusion coefficient obtained by existing experimental technique. Results show that the influence range of edge effect on 100 mm diameter cylindrical concrete specimen is from the outer edge of specimen to the depth of 20 mm inside specimen, while the deviation of chloride diffusion coefficient of concrete increases due to the edge effect in the variable voltage RCM test, hence the accuracy of the tested chloride diffusion coefficient can be improved by obviation of the edge effect on the chloride diffusion depth in concrete specimen.