Abstract: A synthesis route for stable isotope-labeled sulfamethoxazole-benzene ring-¹³C₆ has been developed. In this strategy, ¹³C₆-benzene was used as the source and starting material for stable isotope labeling and starting material to synthesize sulfachloropyridazine-benzene ring-¹³C₆ through 6 steps of nitration, nitroreduction, acylation, sulfonation, condensation and hydrolysis. The product structure was confirmed through characterization techniques such as nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), which the purity of the product was higher than 98.0% and the isotopic enrichment was higher than 99.0atom%¹³C. Sulfachloropyridazine-benzene ring-¹³C₆ can be used as an internal standard reagent for qualitative and quantitative analysis of sulfachlorpyridazine.