【Objective】The Qilian Mountains is the main water head of the HeiheRiver Valley in Northwest China. To balance the need of water between socio-economic development and natural ecology protection, it is very important to understand the hydrology and other scientific basis of water management. Situated at the headwater area of the basin, soil properties especially soil physical properties such as particle size distribution in the Qilian Mountains have significant impacts on hydrological processes. Mode of profile distribution (PDM) of soil particle size composition and its relationships with various environmental factors were discussed in an attempt to further understand quantitatively the transport of water, nutrient, gas and heat in soil and to develop 3D predication and mapping of soil particle size distribution. 【Method】A total of sixty-nine soil profiles ≥1m in depth typical of the Qilian Mountains were investigated and soil samples collected from layers of the profiles for fractionation of clay, silt and sand fractions with the laser diffraction method. Sand fraction, the most variable one, was selected to study mode of its vertical distribution in the profile and its relationships with environmental factors, including elevation, slope, aspect, annual mean precipitation, plan curvature, profile curvature, terrain wetness index, land use and parent materials. 【Result】Results show that the sand fraction in that region had 6 PDMs, i.e., uniform, increasing, decreasing, increasing-decreasing,decreasing-increasing and irregular, which were little related to anyquantifiable environmental factors.We predicted the PDM with random forest, the accuracy was low. It is difficult to predict the PDM with quantifiable environmental information.【Conclusion】Because of the diversity of soil forming parent materials and complex landform and land relief in the Qilian Mountains, migration and discontinuity of parent materials often occurs as a result of land use. So it is difficult to predict PDM quantitatively only with single environmental variables, and therefore, its essential to explore in depth the issue from the angle of composite environmental variables.