Abstract: China's code for seepage tests does not specify the scaling method for oversize treatment, and this problem will disturb testers. The discussion of scaling method for the oversize treatment is of practical significance. The impact of scaling method for the oversize treatment of the results of seepage tests is revealed through laboratory experiments so as to improve the rationality of the test results. The results show that the similar grading method is commonly used in the mechanical strength tests, but not suitable for seepage tests. The equivalent alternative method is only used in the situation that the middle size substitutes the oversize portion. It is suggested that the oversize treatment of seepage tests should keep 30%~40% of fine content unchanged in order to reduce the impact of changes of the fine content on permeability coefficient.