Mud crab (Scylla serrata) is an important commercial crustacean in China. An experiment was designed to study the effect of cold stress on S. serrata. After a one-week adaptation at 28oC, the temperature is suddenly reduced to 4oC. The crabs were sampled every 2 h for 10 h and dissected immediately to measure the enzyme activity. The crabs at room temperature (28oC) were used as the control group. The activity of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX), the content of malondialdehyde (MDA) and the activity of 4 ATPases (Na+, K+-ATPase; Mg2+-ATPase; Ca2+-ATPase; Ca2+, Mg2+-ATPase) were measured biochemically. In contrast to the control group, the SOD activity increased significantly from 2 to 6 h after the cold stress, and then decreased. The CAT and GPX activities increased in 2 h, and then decreased gradually. The content of MDA increased gradually in 4 h. The activity of Na+, K+-ATPase decreased in 2 h, increased up to the top value at Hour 6, then decreased again. The activities of Mg2+-ATPase, Ca2+-ATPase and Ca2+, Mg2+-ATPase increased significantly in 6 h, insignificantly in any other hours. Under cold stress, the activity of antioxidative enzymes in S. serrata was reduced at first then stabilized, ROS-scavenging weakened, and MDA accumulated gradually in the gill after 6 h. The activity of the 4 ATPases in the crab decreased after 6 h, suggesting that the ability to regulate ion concentration has been paralyzed. Therefore, the maximum period to sustain healthy meat in the crab under cold stress is 6 hours. The experiment was designed to study the effect of cold stress on S. serrata. After a one-week adaptation at 28oC, the temperature was suddenly reduced to 4oC. The crabs were The activity of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) was measured at 2 h for 10 h and dissected immediately to measure the activity of the enzyme , the content of malondialdehyde (MDA) and the activity of 4 ATPases (Na +, K + -ATPase; Mg2 + -ATPase; Ca2 + -ATPase; Ca2 +, Mg2 + -ATPase) were measured biochemically. The activity of Na +, K + -ATPase decreased in 2 h, and then decreased decreased. The content of MDA increased gradually in 4 h. The activity of Na +, K + -ATPase decreased in 2 h, increased up to the top value at Hour 6, then The activities of Mg2 + -ATPase, Ca2 + -ATPase and Ca2 +, Mg2 + -ATPase increased significantly in 6 h, insignificantly in any other hours. The cold stress, the activity of antioxidative enzymes in S. serrata was reduced at first then stabilized The activity of the 4 ATPases in the crab decreased after 6 h, suggesting that the ability to regulate ion concentration has been paralyzed. Thus, the maximum period to sustain healthy meat in the crab under cold stress is 6 hours.