The ubiquitin-proteasome system (UPS) is an important system for regulating protein degradation and function. In order to evaluate the effects of acute crowding stress on flesh quality and UPS in fish, we randomly distributed (initial body weight:56.88 g±10.54 g) into 6 aquarium tanks (length:200 cm, wide:100 cm, depth of water:55 cm) at two stocking density (three replicates for each group). Water temperature (17.0±0.5)℃, pH (7.5±0.3) and dissolved oxygen (7.0 mg/L) were maintained at the same level in each group. Fish serum and muscle samples were collected at 0 h, 1 h, 6 h, 12 h, 24 h, and 48 h after crowding stress as well as at 6 h and 168 h after the crowding stress was removed (treatment group stocking density changed from 100 kg/m² to 10 kg/m²). The serum cortisol, muscle texture, the mRNA expressions of , and genes related to UPS as well as the content of ubiquitinated protein in fish muscle were investigated. The results showed that the serum cortisol level significantly increased after acute crowding stress, suggesting the crowding environment led to fish stress. The serum concentration of cortisol recovered to the control level after 168 h of recovery. Crowding stress also exerted an influence on muscular texture characteristics of cultured grass carp. Muscular hardness and cohesiveness decreased significantly in grass carp suffering from cute crowding stress, and the two flesh quality indexes recovered to the level of the control group after 168 h of post-stress recovery. However, muscular adhesiveness, chewiness and resilience were significantly higher than those in the control group at the end of the experiment. Compared with the control group, the mRNA expressions of in muscle of fish stocked at the treatment group were significantly increased after 48 h of crowding stress, and then returned to the control level after 168 h of post-stress recovery. It is implied that the acute crowding environment might cause cellular stress in fish. In the UPS pathway, the mRNA levels of were significantly increased after crowding stress. The transcriptional levels of these genes did not returned to the control level after 168 h of post-stress recovery. Moreover, a significant higher content of ubiquitinated proteins occurred in the dorsal muscle of fish in the treatment group at 6 h and 12 h after crowding stress. Subsequently, there were no significant difference in ubiquitinated proteins of fish stocked at two densities, while ubiquitinated proteins in the treatment group significantly declined compared to the control group at 168 h of recovery. It was concluded that the acute crowding environment induced the stress response and altered the muscular texture as well as improved muscle UPS activity. The deleterious effects of acute crowding stress on fish physiological function and muscular texture were reversible. Post-stress recovery time should be extended more than 168 h so that the stressed fish reestablish the homeostasis on UPS.