The high accuracy mass number of fragmentation fragments(resolution>70 000,m/z 200) was obtained by quadrupole/electrostatic field orbitrap high resolution mass spectrometry(Q-Orbitrap-HRMS),and the element composition was obtained by element simulation,and then the fragmentation law for Sudan red Ⅰ,Ⅱ,Ⅲ and Ⅳ was investigated.Results showed that the existence of hydroxyl groups in molecular structure could easily be protonated to form ［M+H］+,and the cracking process occurs in the vicinity of azo groups:First,the presence of azo-conjugated o-phenol hydroxyl group(H group) leads to rearrangement of compound structure from enol to hydrazone,where H is transferred to N atom,and the high bond azo bond(—N‖N—) is rearranged to nitrogen single bond(—NH—N‖).Second,no migration occurs as the azo bond is ortho free for H,while the azo bond(—N‖N—) with high bond energy remains unchanged,and the fragment ions are formed by breaking the C—N bonds on both sides.Third,in high impact energy,there exist simultaneous cleavage of the chemical bond and open-loop cleavage of the ring-ring,and the fragment ions containing a plurality of azo groups are also richer.The best ionization mode and mass spectrum fragmentation law for the four Sudan Red dyes were determined by direct injection,which provided a basis for the rapid identification of such compounds.