The invention discloses a ductile iron bar with a small diameter smaller than 25 mm. The ductile iron bar with the small diameter smaller than 25 mm comprises, by weight, 3.4%-3.8% of C, 2.4%-3.3% of Si, no more than 0.4% of Mn, no more than 0.10% of P, no more than 0.06% of S, and the balance Fe, and the sum of the weight percents of all the components is 100%. The invention further discloses a preparation method of the ductile iron bar. The ductile iron bar with the small diameter smaller than 25 mm is compact in structure, free of shrinkage defects such as shrinkage cavities and porosity, moderate in hardness (radial carbides are avoided, and the content of free carbides is smaller than 10%), smooth in surface, and high in strength. The strength index of a body sample meets the LZQT500-7 requirement, the cutting machinability is good, the cutting resistance is small, high-speed finish machining can be achieved, and relative fluctuation of the smoothness of a machined surface is small.