<p>The crystallinity has the potential to distinguish the primary and secondary calcite in Chinese loess, which then provides insights into illuviation depth and variations of the East Asian Summer Monsoon. However, this aspect has been rarely investigated. In this study, we defined the crystallinity of calcite as the height/area (<i>H</i>/<i>A</i>) ratio of the diffracted peak at crystal face (1 0 4). The <i>H</i>/<i>A</i> ratio inversely correlates with the average width of the diffracted peak, where a higher <i>H</i>/<i>A</i> ratio indicates higher crystallinity of calcite. Through the mixing and synthetic experiments, we found that the <i>H</i>/<i>A</i> ratio is minimally affected by factors such as calcite content, deposition temperature or rate but significantly influenced by the ionic impurity and the mixing proportion of different calcites. Subsequently, we examined desert samples of loess sources and loess carbonate nodules. Desert samples predominantly consist of primary calcite which inherits characteristics from cryptocrystalline limestone with high levels of ionic impurities resulting in low <i>H</i>/<i>A</i> ratio of 4.30 ± 0.51. In contrast, loess carbonate nodules contain abundant secondary calcite precipitated within soil interstices with low levels of ionic impurities leading to a significantly higher <i>H/A</i> ratio of 7.76 ± 0.82. Consequently, higher <i>H</i>/<i>A</i> ratios during interglacial periods compared to glacial periods are attributed to variations in relative proportions between primary and secondary calcite in loess sequences. The thickness, between the glacial-interglacial boundary and the depth where the <i>H</i>/<i>A</i> ratio starts to increase from the bottom to the top in the loess layer, can be used to indicate the illuviation depth of upper-soil carbonates and the intensity of the East Asian Summer Monsoon. This proxy can be further applied in long-term loess sequences to uncover the summer monsoon evolution.</p>