Abstract: A certified reference material of thiamethoxam in acetonitrile was developed and approved as a primary certified reference material (GBW08772). It was prepared by gravimetric-volumetric method with value assigned thiamethoxam material (purity 99.4%) and chromatographic grade acetonitrile as solvent. Research has found that thiamethoxam could undergo a guanidyl degradation in water and methanol to produce thiamethoxam-urea, therefore acetonitrile was selected as the solvent. According to JJF 1343-2022, the solvent stability, homogeneity, short-term stability, and long-term stability tests were carried out for the prepared solution using high-performance liquid chromatography. Finally, the mass concentration of the developed reference material GBW08772 was 1.00 mg/mL with a relative expanded uncertainty of 1% (k=2). The developed solution certified reference material of thiamethoxam has traceability and accuracy, and meets the requirements of national food safety determination.