Abstract: Fasting or starvation is a common method used in the cultivation of mud crabs (Scylla paramamosain) to regulate their substance and energy metabolism, achieve efficient wintering, and promote off-season sales. To explore the effects of fasting duration under low-temperature conditions on the growth and nutritional composition of mud crabs, a group of first-year female crabs (180±20 g) were subjected to a fasting experiment at a water temperature of (14±1) ℃. Samples were taken at 0, 2, 14, and 60 days of fasting to compare and analyze the impact of fasting duration on the nutritional composition of the edible parts (muscle and hepatopancreas) of the crabs. The results showed that, ① under low-temperature conditions, fasting didn’t lead to crab mortality and had no significant impact on weight, carapace length, width, and height. However, the hepatosomatic index and condition factor gradually decreased with increasing fasting duration (P<0.05). ② Fasting didn’t significantly affect the conventional nutritional components of the muscle, but the crude lipid content in the hepatopancreas decreased significantly with fasting duration, while the moisture, crude protein, and ash content increased (P<0.05). ③ In terms of amino acid composition, 17 amino acids were detected in both the muscle and hepatopancreas. Except for slight changes in the proportion of individual amino acids, the proportions of most amino acids remained stable. ④ During fasting, the crabs notably retained a high level of unsaturated fatty acids, such as 20:4n6 (ARA) and 20:5n3 (EPA), while the retention level of 22:6n3 (DHA) was low. C16:0 and C16:1n were the main energy-providing substances. The results suggest that fasting significantly alters the nutritional composition of the hepatopancreas compared to muscle tissue, possibly by inducing the metabolism of fatty acids, particularly SFA, to provide sufficient energy. This indicates the vital role of SFA in the wintering of Chinese mitten crab, and supplementation before wintering may promote more efficient wintering, while post-wintering fat storage may require enhanced SFA nutritional supply. The study enriches our knowledge of energy and nutritional metabolism in crabs and provides a preliminary research foundation and theoretical basis for further studies on precise and efficient wintering and post-wintering fattening of S. paramamosain.