The microstructure of rear axle main reducer shell bearing seat material QT450 with 25% P + 75% F and 50% P + 50% F was compared to study the internal relation between structure and fracture splitting perfor- mance． Firstly，tensile tests were performed on the QT450 specimens with different metallographic structure to ob- tain the material parameters required for numerical simulation． Combined with the simulation，the critical fracture toughness J IC of the two matrix tissues was determined respectively． The fracture-splitting force and the stress con- centration of QT450 with different microstructures were obtained by ABAQUS． Then a springback model was estab- lished to analyze the plastic deformation and out-of-roundness of the two tissues． Finally，the fracture splitting test was carried out to verify the validity of the numerical analysis． The results show that QT450 with 50% pearlite + 50% ferrite is more suitable for the fracture splitting than QT450 with 25% pearlite + 75% ferrite． The strength and stiffness of QT450 material depend on the content of pearlite． With the increase of pearlite content，the britt- leness of the material，the notch sensitivity increase and the fracture splitting performance are all improved． The shell bearing seat produced by QT450 with 25% pearlite + 75% ferrite matrix has an out-of-roundness of more than 0. 2 mm，which can not complete the subsequent finishing and assembly． While the shell bearing seat pro- duced by QT450 material with 50% pearlite + 50% ferrite has an out-of-roundness of less than 0. 2 mm，and the plastic deformation caused by fracture splitting is small，so it can well meet the requirement of the actual produc- tion process．