Abstract: During the assembly process of a certain rotor support seat, it was found that it had broken. The reasons for the fracture of the rotor support seat were studied using methods such as macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and mechanical performance testing. The results show that under the action of assembly bending stress, the rotor support seat experienced brittle overload fracture mainly characterized by cleavage fracture. The presence of casting defects on the fracture surface resulted in stress concentration, which reduced the strength and plasticity of the material. The high carbon element content and coarse grain size of the rotor support seat led to an increase in pearlite content and a decrease in ferrite content in the material, resulted in the appearance of widmanstatten structure and coarse subgrains in the material, ultimately led to brittle fracture of the rotor support seat.