This paper investigates the effects of chilled iron on the microstructure and properties of QT400 ductile iron, including the presence or absence of chilled iron and the type of chilled iron material. It introduces the current application status of ductile iron and the classification as well as the precautions for using chilled iron. Through experimental comparisons, the microstructure and mechanical properties of QT400 ductile iron under three conditions—without chilled iron, with gray cast iron chilled iron, and with ductile iron chilled iron—are analyzed. The results indicate that the addition of chilled iron increases the number of graphite nodules and promotes their uniform distribution, which contributes to an enhancement in tensile strength, yield strength, elongation, and low-temperature impact toughness.