Abstract: The defects of suspected cracks in the heart of 40CrNiMoA steel used for crankshaft of an automobile engine were found during the low magnification acid corrosion test. The causes of defects were analyzed by means of acid etching contrast, chemical composition analysis, metallographic examination and magnetic particle inspection. The results show that the core microstructure of the material was not dense, and the abnormal bainite structure in the core led to its non-corrosion resistance. Corrosion pits were generated in the acid corrosion process, and these corrosion pits were linked together to form a "pseudo-crack" defect. After optimizing the process parameters of the continuous casting and rolling process of the material, the density was increased, the abnormal bainite microstructure in the heart disappeared, and there was no obvious corrosion pit in the heart after 30 min of acid etching.