<p>To improve the forming quality of the diesel engine flywheel and effectively solve casting defects, the solidification process and microstructure were investigated using a coupled finite element and cellular automaton (CAFE) approach. The optimal casting parameters, determined through orthogonal testing with the minimal defect rate as the criterion, were a pouring temperature of 1480°C, a mold temperature of 25°C, and a pouring time of 33 s. Simulations based on the optimal parameters predicted a shrinkage porosity volume of 0.808 cm³, compared to 4.043 cm³ observed in conventional approaches, and the defect rate was reduced by 79.9%. Furthermore, the microstructure of the ductile iron flywheel was simulated by the CAFE module to study the effect of the surface and bulk nucleation parameters. The results indicated that the surface nucleation parameter had a negligible influence on the solidification microstructure. In contrast, the volume nucleation parameter exerted a considerable influence. As the average bulk undercooling (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\Delta }T_{{v,{\text{max}}}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Δ</mi> <msub> <mi>T</mi> <mrow> <mi>v</mi> <mo>,</mo> <mtext>max</mtext> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation>) increased, the columnar grain zone expanded while the equiaxed grain zone shrank. Conversely, with increasing maximum bulk nucleation density (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({n}_{v,\text{max}}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>n</mi> <mrow> <mi>v</mi> <mo>,</mo> <mtext>max</mtext> </mrow> </msub> </math></EquationSource> </InlineEquation>), the equiaxed grain zone expanded while the columnar grain zone shrank. This study provides valuable theoretical guidance for controlling casting defects and optimizing microstructural characteristics.</p>