Abstract: Abstract: The parameter spatial aggregation has signi?cant impacts on the results of Annualized Agricultural Nonpoint Source Pollution model (AnnAGNPS). Parameter spatial aggregation mainly includes spatial aggregation of topographic (T) parameters and soil & land use (SL) parameters. The previous studies mainly focused on the coupling impacts of the 2 kinds of parameters' spatial aggregation on the hydrologic process, lacking the separate impacts of the 2 kinds of parameters' spatial aggregation on the hydrologic process in different time scales, and the comparison of the impacts of 2 kinds of parameters' spatial aggregation on hydrologic process. The objective of this study was to distinguish the impacts of T parameters aggregation and SL parameters aggregation on the simulation results of the AnnAGNPS. Taoxi watershed which is located in the southeast of China was selected as the study area. Based on the sensitivity analysis of parameter by differential sensitivity analysis (DSA) method, the AnnAGNPS model with the daily time step was set up. Nine discretization scenarios including 15, 30, 50, 70, 100, 150, 200, 250 and 300 hm2 were established, and 6 topographic indices (i.e., channel length, slope length of subbasin, average slope of channel, maximum gradient of channel, average slope of subbasin and maximum gradient of subbasin) and the areas of different soil and land use types were calculated to evaluate the impact of cell division on the parameter aggregation. And then 7 combination scenarios were carried out to evaluate the impact of T and SL parameters spatial aggregation on runoff and sediment yield at annual and daily time scale, the scenarios T15SL15, T15SL50, T15SL100 and T15SL300 with the same T parameters aggregation were used to analyze the effect of SL parameters spatial aggregation on hydrologic process, and the scenarios T15SL300, T50SL300, T100SL300 and T300SL300 with the same SL parameters aggregation were used to analyze the effect of T parameters spatial aggregation on sediment yield and runoff. The results showed as follows: 1) The aggregation effects of T and SL parameters significantly increased with the increasing of critical source area (CSA). 2) The spatial aggregation of SL parameters reduced the annual runoff and annual sediment yield, and the predicated sediment yield decreased at a greater rate compared to annual runoff responding to the increase in values of CSA; while the T parameters aggregation reduced the annual sediment yield, but had not significant influence on the annual runoff. 3) Aggregation of T parameters was the main factor affecting the daily runoff process, resulting in the increase of peak runoff and the decrease of the time reaching the peak runoff, and its influence on the annual maximum one-day runoff was greater than the annual maximum five-day runoff. 4) The spatial aggregation of SL parameters had great effects on annual maximum one-day and annual maximum five-day sediment load, and the annual maximum one-day and annual maximum five-day sediment load decreased significantly with the increase of aggregation degree. The change ratio of the annual maximum five-day sediment load was a little larger than that of the annual maximum one-day. Additionally, the T parameters aggregation resulted in the decrease of the time reaching the peak sediment.