Abstract: Analysis on the fracture failure reason of the Q235B steel lower connecting rod through the methods such as chemical composition test,hardness testing,impact performance testing,tensile testing,metallographic structure and fracture morphology analysis.The analysis results show that the carbon element of the Q235B part exceeds the stand requirement,and the hardness diatribution aroud the crack defect is uneven and higher than the standard requirement,and the impact performance of the failure part is far less than the stand requirement,The microstructure distribution around the defect is uneven,and the fracture morphology presents cleavage fracture morphology,indicating brittle fracture.The uneven distribution of the microstructure and mechanical properties of the failure part and more defects in the core part,are the main reasons for the cracking of the failure part.