A new self-designed mechanochemical reactor, inlaid pan-mill, was used in studying high density polyethylene (HDPE). The effects of pan-milling stress on the structure and properties of HDPE were investigated. Gel permeation chromatography, melt indexer, Fourier transformed infrared spectroscopy, electron spectroscopy for chemical analysis, differential scanning calorimetry, X-ray diffraction, capillary rheometer, and Instron material testing system were used to characterize the structures and evaluate the properties of HDPE. The results showed that mechanochemical degradation of HDPE occurred under the stress fields of pan-mill, the molecular weight of HDPE was reduced, and HDPE with higher initial molecular weights were easier to degrade under the stress fields. Oxygen-containing groups such as COOH, C=O, and C-O were introduced to HDPE chains as a result of degradation during milling. Crystallinity of HDPE first decreased slightly followed by gradual increases with increasing mining times; monoclinic crystals appeared after four cycles of milling and increased markedly with increasing milling times. Pressure oscillation in capillary flow occurred at significantly higher shear stress and shear rate for milled HDPE than unmilled HDPE. After milling, mechanical properties were improved.