Soil is the product of rock weathering, and the interplay between rocks and soil is a fundamental aspect of Earth System Science with direct relevance to human activities. To explore the material composition and evolutionary dynamics of the rock—soil system, it is imperative to select in- situ weathered areas for comprehensive soil vertical profile sampling. Drawing upon data from the Hainan Island Regional Ecological Geological Background Survey Project, this article presents a scientifically grounded approach for identifying eluvium profiles. In addition, the paper summarizes the vertical distribution law of chemical elements in the rock—soil system of the basalt formation in northern Hainan. By utilizing the geochemical data from the vertical profiles of Hainan basalt, the following conclusions are drawn: The SiO2 content in the vertical profiles shows a characteristic of high values in the A layer and rock, and low values in the B and C layers, which is opposite to the trend of Al2O3 content. There is an inverse relationship between these two representative components. Other components follow the same trend based on their affinity, while certain trace elements that are significantly influenced by surface activities, such as organic matter and I, do not conform to this pattern.