Abstract: This study investigated the effects of ultrasonic treatment on the quality characteristics of postmortem mutton from the longissimus dorsi muscle of Lop sheep during ripening. Ultrasonic treatment was applied at 250 W and 22 kHz for 30 min at 4 ℃. The meat was then aged at 4 ℃ for up to 5 days, and quality parameters were evaluated at 0, 1, 2, 3, 4, and 5 days. Analyses included (Total Volatile Base Nitrogen, TVBN), total viable count, pH, color, water-holding capacity, texture profile, myofibrillar protein degradation, microstructure, and volatile flavor compounds. The results indicated that TVB-N values and total viable counts increased with prolonged aging time. pH increased after 1 day of aging, while shear force and hardness significantly decreased (P<0.05). The L^* value reached its maximum on day 3. Cooking loss was significantly reduced after 3 days of aging (P<0.05). Ultrasonic treatment accelerated the pH increase, reduced cooking loss, and resulted in substantially lower TVB-N and shear force values compared to the untreated control after 3 days (P<0.05). SDS-PAGE analysis revealed gradual degradation of high-molecular-weight proteins and an increase in low-molecular-weight protein content, which contributed to improved tenderness. Transmission electron microscopy and Hematoxylin–Eosin staining showed that sarcomere length increased during aging, while muscle fiber diameter and cross-sectional area decreased. Ultrasonication significantly increased sarcomere length (P<0.05), increased muscle fiber density, and promoted structural loosening of the muscle tissue. A total of 82 volatile flavor compounds were identified at 0 and 3 days of aging. Key aroma compounds included hexadecanal, ethyl palmitate, nonanal, octadecanal, ethyl tetradecanoate, 2-undecenal, and tetradecanal. The aldehyde content increased significantly by day 3 in the ultrasonicated group, suggesting enhanced flavor development. In conclusion, ultrasonic treatment improved the tenderness and overall quality of mutton, with optimal quality observed on the third day of aging.