Abstract: Planktonic foraminifera is one of the important materials that can provide the information of the physico-chemical environments. The development of in-situ microscopic analysis technology provides a new opportunity to carry out the detailed and intuitive understanding of the distribution and variation of trace elements in biological shells. At the same time, it also promotes the rapid development of paleoenvironmental inversion related research by using the element contents and ratios of the foraminifera shells microregion. In this paper, we used electron microprobe analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and scanning electron microscopy (SEM) to conduct in-situ geochemical analysis of Mg/Ca in the four planktonic foraminifera shells of Globigerinoides ruber, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotalia inflata in the surface sediments of the northeastern slope of the South China Sea. The results of electron microprobe mapping showed that G.ruber had periodic bands with high Mg contents, which might be associated with symbionts. However, N.dutertrei, P.obliquiloculata and G.inflata had thicker calcite crusts with low Mg contents. The results of LA-ICP-MS also showed that there were obvious low Mg/Ca calcite crusts with of N.dutertrei, P.obliquiloculata and G.inflata, while multiple sets of Mg/Ca ratios can be seen in the shells of G. ruber, which were consistent with the results of electron microprobe mapping. The distribution of Mg/Ca ratios in shells were affected by the biomineralization process of foraminifera, and the presence of contaminants also led to higher Mg/Ca ratios. Therefore, we conclude that the large variations of Mg/Ca ratios in foraminifera shells are not only affected by the surrounding seawater temperature, but also constrained by other factors. In addition, we found that reliable Mg/Ca ratios can be obtained by the LA-ICP-MS test. The application of this method can avoid the complex pretreatment process, and provide a technical basis for efficient, fast, high spatial resolution and low sample volume testing in the future.