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Correlating morphological surfaces, such as terraces and alluvial fans, is necessary in geomorphology researches. Gravel characteristics have been analysed to estimate the weathering degree. However, the composition of gravels was complicated and a feasible quantitative method is still need. Therefore, the objective of this study was to investigate the weathering process of an alluvial fan in western Sichuan Basin and find a practical method in correlating morphological surfaces. The Qingyi River frequently migrated since 500 ka ago and generated several paleo alluvial fans, one of which is the Dansi alluvial fan. Major and trace element samples form an uninterrupted outcrop of the Dansi fan were tested to estimate the weathering process of a single fluvial profile. Results showed that the weathering index decreased regularly with depth, meaning that the surficial sample could represent the weathering degree of the morphological surface. In addition, the sand fillings between the gravels in the river sediments were sensitive to the weathering process and were able to reflect quantitative weathering degree. Therefore, we suggested that the weathering index of the sediments closed to the surface can represent the weathering degree of morphological surfaces.