To elucidate diagnostic horizon characteristics of the soils derived from loess in subtropical environment on the southern slope of the Qinling Maintains and their significance in soil classification, a soil profile was selected on the first fluvial terrace in Qianfang Village of Yunxuan County, the upper reaches of the Hanjiang River, for investigation of profile structure and analysis of particle-size distribution, magnetic susceptibility, major elements, minerals and mcromorphology. Results show that the soil distributed in the Hanjiang River Valley has an A-AB-Bt₁-Bt₂-BC-C profile structure. The 90～230cm horizon (Bt) of the soil profile is clayey loam in texture and prismatic-bloky and compact in structure, and its structural planes are covered with red brown Fe-Mn clay cutan. Microscopic observation reveals that this horizon is dominated with illuvial clay, mostly in the form of cutan and various shapes of optically oriented fibrous clays, too. Its clay (<2μm) content reaches 18.81%, which is 1.76 times that in the overlying leached layer (10.70%), and 12.57 times that in the underlying parent material layer (7.32%), suggesting that the Bt horizon features an Argic horizon and the diagnostic characteristics of a clay pan and is comparable with the typical Calypani-Udic Argosols in China in argillization degree. Its clay minerals are dominated by illite and some kaolinite. The soil is still at its late decalcification stage of the weathering process, has not yet any significant sign of allitization, and is moderately weathered. The soil can be sorted as Calypani-Udic Argosol.