Abstract: Through quantitative analysis of alloy composition, microstructure, fracture observation and measurement of mechanical properties, the large deformation and uneven fracture after fracture defects of the medium carbon microalloyed non-quenched and tempered steel 46MnVS5 connecting rods were analyzed. The results show that the tensile strength of the two connecting rods reaches 1120.8 MPa-1140.2 MPa, which is close to the upper limit of the standard; the area reduction of 1^#connecting rod exceeds the standard requirement, which is 47.2%-47.7%. The plasticity of 2^#connecting rod meets the standard, and the area reduction is close to the upper limit of the standard, reaching 43.2%-44.7%. The strong plastic volume reaches 18052.9-19743.9 MPa·%. In terms of the performance, the reduction of area exceeds the standard, and the strong plastic product is too high, which is the main reason for the fracture defects. It is proposed that using thermomechanical control process（TMCP） to precisely control the microstructure and properties, which is the key to solve the defects. It provides theoretical support for improving the quality of connecting rods.