【Objective】Soil moisture content is one of the major factors determining degrees of soil aggregate breakdown, which alters the distribution of particle sizes in sediment. Soil surface hydrological condition reflects the spatial distribution of soil moisture content at slope, and strongly affects the characteristic of soil aggregate loss. However, previous researches focused on the influence of soil surface hydrological condition on runoff and sediment yield process within flat tillage system, and limited information are available about effect of soil surface hydrological condition on soil aggregate loss, especially under seepage condition in a contour ridge system. While, contour ridge system is an effective soil conservation practice used worldwide.【Method】In this study, simulated rainfall experiment was conducted to determine the lost characteristic of six size soil aggregates(≥5, 2-5, 1-2, 0.5-1, 0.25-0.5, and<0.25 mm) under three soil surface hydrological conditions (free drainage, soil saturation, and seepage) and three rainfall intensities (30, 60, and 90 mm•h-1)for cinnamon soil in a contour ridge system.【Result】Results showed that soil aggregate loss was in order of seepage>soil saturation>free drainage, seepage and soil saturation had 1.44-4.22 and 0.26-3.12 times soil aggregate loss greater than free drainage condition. Contribution rate of the existence of soil saturation and seepage for soil aggregate loss was significantly 2.02-3.71 and 1.28-2.51 times higher than that of water potential difference with the change of rainfall intensity. There was significantly difference in the loss distribution of soil aggregate among soil surface hydrological conditions. Soil saturation and seepage resulted the loss percentage of 2-5, 1-2, and 0.5-1 mm aggregate increased by 46.74%-121.19%, but weakened more than 90% of 0.25-0.5 mm aggregate loss. In addition, soil saturation and seepage increased by 23.77%-215.80% for the enrichment rate of >0.5 mm aggregate, but induced the enrichment rate of 0.25-0.5 mm aggregate decreased by 45.07%-68.90%. Because 0.25-0.5 mm aggregate occurred re-aggregate during erosion process, and the transport capacity of 2-5 mm aggregate was strengthened under the interaction of soil saturation and seepage. For the four characteristic indices of lost soil aggregate (the mean weight diameter, geometric mean diameter, mean weight soil specific area, and fractal dimension), difference in the mean weight diameter was the largest among soil surface hydrological conditions. The mean weight diameter was better indicator characterizing soil aggregate loss under free drainage and soil saturation conditions, while it was fractal dimension under seepage condition. Soil aggregate loss pronouncedly increased with the increase of rainfall intensity under free drainage, soil saturation, and seepage conditions. Among, effect of rainfall intensity on soil aggregate loss was the most pronounced under free drainage condition, and rainfall intensity had the greatest influence on 2-5 mm aggregate.【Conclusion】Soil saturation and seepage significantly enhanced soil aggregate loss, and showed more pronounce influence on the loss of lager size aggregate. These findings enhance our understanding of erosion mechanism for soil surface hydrological condition and contour ridge influence on soil erosion process, and supply guidance for implementing contour ridge system.