Abstract: The early fracture of the tie bar of a large-scale injection molding machine occurred during service. The fracture cause was studied by fracture morphology observation, chemical composition analysis, non-metallic inclusion analysis, microstructure observation and mechanical property test, and some improvement measures were put forward. The results show that the fracture characteristic of the tie bar was ductile fracture. Under the combined action of thermal stress, microstructure stress caused by improper subsequent heat treatment and continuous reciprocating tensile force, the cracks initiated from the stress concentration of shrinkage cavity residue and banded segregation in the core, and propagated, finally leading to the fracture of the tie bar. After strictly controlling the chemical composition of raw materials, properly increasing the normalizing temperature, increasing the forging ratio, properly prolonging the preheating holding time, and properly prolonging the quenching holding time, such failure did not happen again.