Abstract: To investigate the effects of superfine grinding on the quality characteristics of hawthorn seed, this study treated hawthorn seed by superfine grinding with different time gradients, and characterized the changes in physicochemical and functional properties including particle size, color, hydration properties, and flowability of the powder. Spectroscopic methods were applied to determine the effects of different superfine grinding times on the total flavonoid extraction rate and antioxidant capacity, identification of flavonoid components in hawthorn seeds using high-performance liquid chromatography-tandem mass spectrometry. The microstructural and morphological changes were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The results demonstrated that the particle size of hawthorn seed decreased significantly after superfine grinding (P < 0.05), but the uniformity of the powder deteriorated. The powder obtained at 20 min of superfine grinding exhibited the relatively good overall quality, under which the median particle size (D₅₀) was substantially reduced to 14.38 μm, representing a marked decrease from 35.67 μm in the coarse powder. The water-holding capacity and oil-holding capacity increased from 1.67 g/g and 0.67 g/g in the coarse powder to 2.34 g/g and 0.95 g/g, respectively, at 20 min (P < 0.05), representing substantial improvements of approximately 40.1% and 41.8%. The solubility also increased progressively with grinding time and reached a plateau at 20 min, with no statistically significant difference observed between 20 min and 25 min. In contrast, the angle of repose increased markedly from 15.81° to 32.07° and the angle of slide from 35.35° to 52.37°, revealing a progressive decline in powder flowability with extended grinding. The total flavonoid extraction rate increased significantly from 1.22 mg/g in the coarse powder to 1.73 mg/g at 20 min (P < 0.05), representing a 41.8% improvement, and further to 1.75 mg/g at 25 min, although the increment beyond 20 min was markedly diminished. Correspondingly, the DPPH radical scavenging activity and ABTS cation radical scavenging activity of the extracts both increased consistently with grinding time, demonstrating enhanced antioxidant capacity. A total of 19 flavonoid components were successfully identified by high-performance liquid chromatography-tandem mass spectrometry, including quercetin, myricetin, catechin, epigallocatechin, and various flavonoid glycosides, and their presence was closely and significantly associated with the observed enhancement of antioxidant activity. Microstructural and structural characterization revealed that superfine grinding for 20 min effectively disrupted the intact cell wall matrix of hawthorn seed, promoting the progressive deconstruction of amorphous cellulose and partially crystalline cellulose within the superfine powder, and consequently leading to a significant reduction in the crystallinity index. X-ray diffraction analysis further confirmed that the characteristic diffraction peak of cellulose type I at approximately 22° remained unchanged across all grinding times, indicating that the fundamental crystalline polymorph was preserved despite the mechanical treatment. Fourier transform infrared spectroscopy revealed no emergence of new functional groups or notable shifts in absorption peaks among different grinding times, confirming that superfine grinding did not alter the molecular backbone or introduce new chemical bonds. Scanning electron microscopy observations at both low and high magnifications provided direct visual evidence that the coarse powder particles were large, irregularly shaped, and exhibited rough surfaces with sharp edges, whereas superfine grinding progressively reduced particle dimensions, produced more uniformly shaped particles with rounded edges, and generated numerous surface micro-cracks and porous structures. In conclusion, superfine grinding significantly improved the quality characteristics of hawthorn seed powder, and the powder obtained at a grinding time of 20 min exhibited the optimal physicochemical and functional properties, demonstrating that superfine grinding is a highly effective and promising technology for the high-value utilization of hawthorn seed as a food processing by-product.