During the solidification of heavy section ductile iron, the slow cooling rate in the central region tends to reduce the number of graphite nodules, form fragmented graphite, and cause shrinkage defects, significantly weakening its mechanical properties. To solve this problem, a novel capsule-based slow-release inoculation process was proposed. Through capsule-type slow-release inoculation, the release of inoculants is delayed to optimize the graphite morphology and increase the number of graphite nodules. Three groups of samples (HDQT-0, HDQT-0.1, and HDQT-0.3) were prepared via a composite process combining in-ladle impingement spheroidization inoculation with slow release inoculation. The effects of inoculation methods on the morphology and mechanical properties of graphite were comparatively analysed. The results reveal that the capsule-based slow-release inoculation process significantly increases the graphite nodule density in the central region of heavy sections from 72/mm2 to 100/mm2, improves the nodularity rate from 70.86% to 81.52%, and reduces the average graphite diameter from 20.51 μm to 18.74 μm. By increasing the number of graphite nucleation sites, the process refines the matrix grains and homogenizes the carbon diffusion pathways, effectively suppressing the formation of fragmented graphite. Mechanical property tests indicate that the tensile strength and elongation increase by 8.45% (from 367 MPa to 398 MPa) and 29.82% (from 15.02% to 19.5%), respectively.