Abstract: The drop-dregs defect is the main quality problem of C70S6 non-quenched and tempered steel fracture splitting connecting rod.In order to explore the causes of drop-dregs, the chemical composition and content of two C70S6 connecting rods with and without drop-dregs were compared and analyzed. The fracture morphology, non-metallic inclusions and microstructure of the steels near the fracture were observed and analyzed by means of metallographic microscope and scanning electron microscope.The results show that the content of N in the 1^#(drop-dregs)steel is low.Properly increasing the content of N is conducive to improve the expansion and fracture performance of connecting rod. The non-metallic inclusions in the connecting rods are mainly MnS.The inclusions in the 2^#connecting rod are smaller and more numerous than those in the connecting rod 1^#, especially at the end of the fracture, and it causes the plasticity of the 2^#(without drop-dregs)connecting rod is lower than that of the connecting rod 1^#, and it is conducive to fracture splitting. The microstructure of C70S6 fracture splitting connecting rod is ferrite + pearlite, and 1^# connecting rod pearlite accounts for 97%-99%. There is mixed crystal structure with different grain sizes in the middle area. Too high pearlite content and mixed crystal structure are the main reasons for connecting rod drop-dregs defect.