Abstract: The mud crab (Scylla paramamosain) is a marine species of significant economic value, renowned for its delicious flavor, strong adaptability, and rapid growth. As a euryhaline species, it demonstrates good tolerance to salinity and alkalinity after domestication. Salt-tolerant rice, also known as seawater rice, can grow in saline-alkaline soil and is characterized by its resistance to salt, alkali, flood, lodging, and pest. The integrated farming of salt-tolerant rice and mud crab, a new model that has emerged in recent years, enhances the economic output per unit area of paddy fields. In order to explore the food source of mud crabs in the salt alkali tolerant rice and mud crabs co-culture system, this study released mud crabs into a salt alkali-tolerant rice field at a density of 0.6 individuals per square meter. After a two-month breeding experiment, the food sources of mud crabs within this ecosystem were analyzed using carbon and nitrogen stable isotope technology. The research results showed that the water quality in the paddy field was promising. The mud crabs exhibited a specific growth rate of (2.71±0.27)%/d and a survival rate of (14.26±4.85)%, with mud crab yeild at (116.30±39.59) kg/ha and rice yield at 3332.25 kg/ha. The provision of compound feed to the crabs did not adversely affect water quality. In terms of isotopic composition, mud crabs displayed δ¹³C values of (–21.55±0.90)‰ and δ¹⁵N values of (9.96±0.40)‰. The potential food sources exhibited a range of δ¹³C values from (–30.51±0.25)‰ to (–13.45±0.15)‰. Among these sources, Alternanthera philoxeroides exhibited the lowest δ¹³C value at (–30.51±0.25)‰, followed by Oryza sativa at (–28.73±0.73)‰, while Echinochloa crus-galli exhibited the highest δ¹³C value at (–13.45±0.15)‰. Similarly, the potential food sources displayed δ¹⁵N values ranging from (5.97±0.13)‰ to (11.38±0.11)‰, with compound feed recording the lowest δ¹⁵N value at (5.97±0.13)‰ and flying insects the highest at (11.38±0.11)‰. The study found that compound feed constituted the primary dietary component for mud crabs, contributing (61.2±3.8)% to their diet. This was followed by contributions from Echinochloa crus-galli and Leptochloa chinensis at rates of (8.3±5.2)% and (8.8±6.1)%, respectively, while flying insects contributed the least, at only (0.9±1.4)%. In conclusion, the study suggests that mud crabs thrive in salt alkali-tolerant rice fields, with compound feed emerging as a significant food source for them within this environment. Additionally, plant-based food sources play a crucial role in the diet of mud crabs in such ecosystems.