Abstract: In order to achieve effective and efficient characterization of phenolic acids in Danshen, the negative ion electrospay ionization tandem mass spectrometry (ESIMSn) experiment was applied to investigate the structures and discuss the fragmentation pathway of phenolic acids. It was found that the ion fragment of M-H-H2O- was the characteristic fragment for the monomer danshensu and caffeic acid. The danshensu was more inclined to lose neutral fragment CO2 comparing with caffeic acid. The dimers not only kept the fragmentation pathway of their monomers, but also had their own special fragmentation pathway. Ion M-H-C₉H₁₀O₅^-was the characteristic ion for salvianolic acid A and rosmarinic acid. The fragmentation behavior of rosmarinic acid also demonstrated that the loss of danshensu was much easier than the loss of caffeic acid. The conjugate structure of salvianolic acid F led to the preference of losing fragment CO₂. As to lithospermic acid, the special pathway was the neutral loss of CO₂ in the MS² and further neutral loss of danshensu in MS³. The oligomer salvianolic acid B experienced a rearrangement during ESI-MS/MS experiment to achieve a conjugate structure. All these unusual neutral losses showed that the negative ion mode was a powerful tool for analysis of phenolic acids from Danshen.