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To determine the high-temperature effect on the soil particle composition (SPC) and its mechanism in Nanning, China, a laboratory experiment was designed based on the high-temperature environment caused by forest fires and considering thermal desorption. The effect mechanism was examined based on the soil organic matter and soil minerals (kaolinite). The experimental results indicated that the SPC was not changed before 200°C. With increasing heating temperature, the content of silt and clay decreased, while the content of sand greatly increased after 200°C. The analysis believes that the reduction of the content of organic matter has promoted the reduction of the content of silt and clay to a certain extent. The decrease in the silt and clay content inevitably increased the sand content, but this was also related to the soil minerals (kaolinite), and a reaction occurred producing a cementing substance that absorbed both silt and clay to form new sand. The temperature effect on the SPC was divided into three parts. The first part was observed from 100 to 200°C, while the soil composition was unchanged. The second part was from 200 ~ 400°C. This part is related to the reduction of organic matter content. The third part was determined to be between 400 and 800°C, which was mainly related to clay minerals. The fusion of silt and clay during the formation of new sand resulted in a decrease in its content beyond 400°C. [ABSTRACT FROM AUTHOR]