Abstract: This study aims to investigate the effects of superfine grinding on quality characteristics of hawthorn seed powder. Hawthorn seed was treated by superfine grinding with time gradients. Physicochemical and functional properties were then characterized, including particle size, color, hydration properties, and flowability of the powder. A systematic investigation was implemented to determine the effects of superfine grinding times on the total flavonoid extraction rate and antioxidant capacity. Flavonoid components in hawthorn seeds were also identified using high-performance liquid chromatography-tandem mass spectrometry. The microstructural and morphological variations 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 after superfine grinding (P < 0.05), leading to the deteriorated uniformity of the powder. The overall high quality was obtained in the powder at 20 min of superfine grinding, under which the median particle size (D50) was substantially reduced to 14.38 μm, indicating a marked decrease from 35.67 μm in the coarse powder. The water- and oil-holding capacity increased from 1.67 and 0.67 g/g in the coarse powder to 2.34 and 0.95 g/g, respectively, at 20 min (P < 0.05), indicating substantial improvements of 40.1% and 41.8%, respectively. The solubility also increased progressively with grinding time, and then reached a plateau at 20 min, with no statistically significant difference between 20 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°, indicating a progressive decline in powder flowability with extended grinding. The total flavonoid extraction rate increased from 1.22 mg/g in the coarse powder to 1.73 mg/g at 20 min (P < 0.05), indicating 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 and ABTS cation radical scavenging activity of the extracts increased consistently with grinding time, indicating enhanced antioxidant capacity. A total of 19 flavonoid components were identified by high-performance liquid chromatography-tandem mass spectrometry, including quercetin, myricetin, catechin, epigallocatechin, and various flavonoid glycosides. Their presence was closely related to the high antioxidant activity. Microstructural and structural characterization revealed that superfine grinding for 20 min disrupted the intact cell wall matrix of hawthorn seed, thus promoting the progressive deconstruction of amorphous cellulose and partially crystalline cellulose within the superfine powder. Consequently, the crystallinity index was reduced significantly. X-ray diffraction analysis further confirmed that the characteristic diffraction peak of cellulose type I at 22° remained constant over all grinding times, indicating that the crystalline polymorph was preserved after mechanical treatment. Fourier transform infrared spectroscopy revealed that there was no emergence of new functional groups or notable shifts in absorption peaks among grinding times, indicating that superfine grinding failed to alter the molecular backbone or introduce new chemical bonds. Scanning electron microscopy observations showed that the large and irregularly coarse powder particles exhibited rough surfaces with sharp edges, whereas superfine grinding progressively reduced particle dimensions for the more uniformly shaped particles with rounded edges and numerous surface micro-cracks and porous structures. In conclusion, superfine grinding improved the quality characteristics of hawthorn seed powder at a grinding time of 20 min, particularly with optimal physicochemical and functional properties. Superfine grinding can be expected to serve as a promising technology for the high-value utilization of hawthorn seed as a food processing by-product.