Abstract: 【Objective】The objective of this paper is to study the compositions of soil particles of the collapsing alluvial fan, bulk density, soil porosity, soil water-holding capacity, the organic matter, and to analyze the relationship among them. The spatial variation law of dilapidated granite alluvial fan farmland soil physical properties was also studied which is not only benefit to agricultural land-use planning, but also provide a theoretical basis for soil improvement of the collapsing alluvial fan farmland, thus, it has a profound effect on agriculture and forestry production in mountainous areas. 【Method】 In this paper, laboratory analysis and field survey methods were adopted to study the small watershed ridge collapse Tongcheng County alluvial fan of farmland. Both paddy and upland land use patterns of alluvial fan were selected, at the same time, soil physical properties of 4 areas in the farmland including the top, middle, margin of the alluvial fan and control trial which is outside the alluvial fan were collected, the Pearson correlation coefficient analysis method was adopted to explore the intrinsic link age between various indicators to understand effect of collapse mound on alluvial fan farmland. 【Result】Collapsing erosion causes serious alluvial fan farmland soil desertification and soil structural deterioration. Coarse alluvial fan farmland particulate matter (gravel and sand) were more than 75.89% compared with the control area on average, soil fine particulate matter (silt and clay) reduced by 39.13% on average compared to the control area. Soil organic matter content was reduced, soil organic matter content in alluvial fan soil reduced by 58.70% compared with the control area on average; bulk density increases, relative to the control area increased an average by 0.25 g·cm^-3; soil porosity were less than the control area, and saturation and capillary water holding capacity were significantly lower in the control area. Meanwhile, along the edge of the alluvial fan to fan roof, gravel and sand content in soil showed a gradual increase, while the silt and clay content gradually reduced, bulk density increased, soil porosity reduced, water holding capacity gradually weakened, organic matter gradually decreased. Correlation analysis showed that the soil bulk density was significantly negatively correlated (R=-0.907 **) with organic matter content; soil bulk density, soil porosity and soil water retention characteristics showed a significant negative correlation; total porosity, capillary porosity and soil water retention characteristics showed a highly significant positive correlation. There was no clear correlation between the non-capillary porosity and various physical properties. Each grain class size of soil and soil bulk density and soil water holding capacity index showed a significant or very significant correlation with the total porosity and capillary porosity.【Conclusion】After collapsing erosion occurred in alluvial fan farmland soil particles, organic matter, soil bulk density, porosity and water holding capacity as the representative soil physical properties showed a serious degradation. In addition, the collapse of the post alluvial fan farmland soil physical properties and spatial variation existed, the nearer the top fan, the worse the soil physical structures. There was linkage between the alluvial fan of farmland soil indicators closely linked, especially the relationship between soil particles and other physical indicators, highlighting the improvement of alluvial fan aspects of soil particles from the soil is workable, such as take-off indigenous soil structure and deep plowing method can effectively improve erosion. The result has provided a basis for understanding of farmland soil physical properties of alluvial fan soil, thus laying a foundation for the improvement of soil structure of the collapsing alluvial fan soil, which is important for agricultural production and economic efficiency of agriculture.

Key words: collapse mound, alluvial fan, soil physical properties, diversity law