Abstract: By the methods of metallographic microscope, scanning electron microscope and mechanics experiment, the causes of fracture cracking of high manganese austenitic cryogenic steel during tensile test were analyzed. The results showed that the segregation occurred in the segregation band of high manganese austenitic cryogenic steel caused by the main alloying elements such as Mn and S in the process of the solidification of a casting blank, and as a result there were large amount of striped MnS inclusions in the rolled steel plate. The interface bonding force between MnS inclusions and the steel plate matrix was weak, so the three-dimensional stress formed in necking process of tensile test led to the fracture cracking of tensile samples.