Abstract: Soil with transitional gradation is called transitional soil.The compaction characteristics of transitional fine-grained soil in subgrade filler are different,and its mechanical properties are greatly affected by matric suction.According to the standard ASTM D698-12,the standard compaction test was carried out,and the optimum water content(OWC) and the maximum dry density (ρ_dmax) were obtained.For each sample,the suction of compacted soil was measured using a dew-point water potential instrument(WP4C) and a high-capacity tensiometer(HCT) before and after compaction.The results show that the increase of the clay content in the transitional soil makes the compaction characteristics of the subgrade different.The maximum dry density decreases with the increase of clay content,and the optimum water content increases with the increase of clay content.When the water content of the wet side of the compaction curve is greater than OWC,the matric suction of compacted soil is close to zero.The matric suction of transitional soil also presents transitional characteristics.When the water in transitional soil is completely absorbed by clay particles,the suction-water content of transitional soil can be fitted as a curve of suction-water content of clay.The same results were obtained in the suction measurements of loose soil particles before compaction.