Abstract: A 16MnDR steel low temperature pressure vessel had the crack after repaired weld. The causes of the crack were analyzed by means of macro analysis, residual stress test, metallographic examination, microhardness test and micro analysis. The results show that the crack of the 16MnDR steel low temperature pressure vessel after repaired weld was a typical liquefying crack of weld metal. Improper control of repair operation process made the microstructure of repair welding abnormal, the hardness of repair welding was too high, and the residual stress was too large, which caused the crack to be produced in the interlayer position of multi-layer weld at the bottom of the sealing weld bead.