XRD-Rietveld full-spectrum fitting method broke through many technical limitations of traditional XRD quantitative analyses and is of great significance in solving the quantitative problem of multi-phase mineral system. Our efforts in this study focused on a comparative study on two granite weathering profiles (A and B profile) in Darongshan area in Yulin, Guangxi. These two profiles were developed adjacent with similar climatic and topographic condition but different parent rock lithology. In order to achieve an accurate and reliable quantitative result, a special fitting strategy was set for the granite weathering samples. Analytical results showed that A and B profile contained similar mineral composition as a whole, mainly including Quartz, K-feldspar, kaolinite, illite and plagioclase. Quantitatively, A profile derived from coarse-grained biotite granite was composed by kaolinite (6.05%~44.67%)+illite (15.85%~49.59%) + quartz (29.72%~46.15% ) + K-feldspar (12.04%~22.85%) + plagioclase (24.33%~32.70%). In contrast, B profile derived from fine-grained biotite granite was composed by kaolinite (3.12%~11.47%) + illite (13.95%~31.94%) + quartz (26.60%~58.05%) + K-feldspar (13.70%~43.47%) + plagioclase (17.95%~23.47%). Both Rwp value (less than 15) and gof value (less than 5) indicated that the calculated spectrum fitted well with the original spectrum, and thus the final quantitative data of minerals was reliable. Also, the analytical results were consistent with geological observations. The geological significance of this study lied in providing quantitative evidence to indicate the weathering process of granite-forming minerals, especially for feldspar group minerals which show an evolution path as feldspar → illite → kaolinite. In addition, based on the significant difference in total clay minerals contents between A and B profile, this study suggested that the granite with coarse-grained texture was more susceptible to suffer intense chemical weathering than those with fine-grained texture under the subtropical climatic environment.