Abstract: The unknown white powder samples extracted from a case were analyzed and identified by scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) combined with infrared spectroscopy (FTIR). The elemental composition of the sample was qualitatively and quantitatively analyzed by SEM-EDS technology. It was inferred that the main elements were oxygen, aluminum and sulfur, and the mass fraction ratio of each element was 8∶1∶2. Further combined with the retrieval and comparison of FTIR spectra, it was preliminarily determined that the sample was ammonium alum. In order to verify the reliability of the analysis results, the reaction characteristics of the samples were verified by chemical tests, and the results were consistent with the inference. The results show that the combined use of SEM-EDS and FTIR could efficiently and quickly identify the chemical composition of unknown powder samples. It had the advantages of simple operation and rapid analysis, which could provide scientific basis and technical reference for the analysis of physical evidence in the field of criminal technology.