The existence forms of molecular ion peaks of 18 acid dyes with different structures and properties,and the high accuracy mass number of fragmentation fragments(resolution>70 000,m/z 200) were studied by quadrupole/electrostatic field orbitrap high resolution mass spectrometry(Q-Orbitrap-HRMS),and the element composition was obtained by element simulation,and then the cracking laws of acid red 1,acid blue 9 and acid red 18 were investigated.Results showed that the molecular ion peaks of these dyes were changed into negatively charged ions by removing all Na ions,in which the bivalent excimer ions with multiple sulfonic groups(SO-3) were the base peaks.With the increase of energy,the fracture process mainly concentrated on the single bond of compound branch chain,in which a tautomer of azido-azo existed in the azo group structure,the azo bond(—N‖N—) of the high bond energy was rearranged into a nitrogen-nitrogen single bond(—NH—N‖),and the C—N bond and —NH—N‖bond were easily broken.While with the neutral loss of the small molecule,the characteristic fragment ion of m/z 79955 7(SO-3) was generated.The optimum ionization modes and the existence forms of molecular ion peaks of 18 acid dyes were determined by direct injection.The mass spectrometry cracking law for acid red 1,acid blue 9 and acid red 18 provides a reference for the analysis of electrospray ionization mass spectrometry behaviors of similar dyes.