Abstract: The ultrasonic testing results of a batch of 4130 steel forgings did not meet the requirements of JB/T 5000. 15-2007 standard. There were dense and single excessive defects in the center of the forging steel. The chemical composition analysis, macroscopic test, metallographic examination, scanning electron microscopy and energy spectrum analysis were used to analyze the location of the defects. The results show that there were a large number of MnS and NbC inclusions in the raw materials of 4130 steel forgings. These inclusions divided the matrix continuity of the steel itself, and cracks were formed during the forging process, resulting in unqualified ultrasonic testing results. Finally, the source and harmfulness of MnS and NbC inclusions were analyzed, and the corresponding process improvement measures were found out. The qualified rate of ultrasonic testing of 4130 steel forgings after improvement was 100%.