Abstract: The synthetic method of bis-(o-trifluoromethylphenyl)dithiophosphinic acid has been improved during the synthetic research of various dithiophosphinic acids. The optimized synthesis has been realized with PCl₃ and (o-F₃CC₆H₄)MgBr as starting materials and via a continuous three-step method including Grignard reaction, sulfuration and substitution reaction. The FTIR spectrum of product shows the presence of substituted aromatic ring, C－F bond, P－S single bond and P＝S double bond. The interpretation result of ³¹P NMR, ¹H NMR and ¹³C NMR of the product, as well as MS, is completely in line with the structure of the targeted compound bis-(o-trifluoromethylphenyl)dithiophosphinic acid.