Abstract: During World War II, Japan extensively deployed and abandoned chemical weapons (CWs) in China. After long-term burial and improper disposal including explosion and incineration, the chemical warfare agents contained in these weapons have been released into the environment, posing persistent risks to ecological security and human health. At present, most residual parent compounds of leaked mustard gas (HD) have degraded partially in the environment, making traditional source tracing analysis methods hardly capable of meeting the requirements of pollutant source identification and historical liability confirmation. Therefore, it is crucial to develop a novel analytical method for source tracing and environmental behavior research. Stable isotope analysis technology can realize the tracing of pollutant sources and degradation pathways by precisely determining the stable isotope ratios of specific elements (e.g., carbon and hydrogen) in target compounds, based on their inherent isotope fingerprint characteristics and fractionation behavior during transformation processes. However, compound-specific isotope analysis (CSIA) of mustard gas and its relevant degradation products has not yet been reported. Based on gas chromatography-isotope ratio mass spectrometry (GC-IRMS), this study established an analytical method for the simultaneous determination of carbon (δ¹³C) and hydrogen (δ²H) isotope ratios of individual mustard gas (HD), 2-chloroethyl ethyl sulfide (2-CEES), and 1,4-dithiane and 1,4-thioxane. Through optimization, the optimal analysis parameters were determined as follows: dichloromethane was used as the extraction solvent, an inlet temperature was set to 210 ℃, and the carrier gas flow rate was set to 1.5 mL/min. Method validation results showed that the carbon isotope analysis has a good linear relationship in the concentration range of 20-500 mg/L (R²>0.99), while hydrogen isotope analysis showed excellent linearity in the concentration range of 500-2 000 mg/L. The measurement precisions are better than 0.30‰ for δ¹³C and 5.0‰ for δ²H, respectively. The application of this method used for analysing 29 samples from different sources demonstrated that δ¹³C and δ²H formed unique fingerprint profiles in the two-dimensional isotope space, which were highly sensitive to differences in synthesis processes and can effectively distinguish samples of different sources. This method provides reliable technical support for the source tracing of chemical warfare agents and has high application value in the monitoring compliance of the Chemical Weapons Convention and the remediation of CWA-contaminated environments.