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In many salt marsh ecosystems, crabs are the most prominent members of the macrobenthic community, influencing material cycling and energy flow. Studies on population dynamics and secondary production are important for quantitative evaluation of the ecological function of crabs; however, few studies have been conducted in salt marsh ecosystems. In this study, we investigated the secondary production and population dynamics of two Thoracotremata species in an Asian salt marsh. A year-round sampling on a monthly basis in the Dongtan salt marshes of the Yangtze River estuary yielded 12 species of crabs belonging to 7 families and 10 genera. The dominant crabs were Chiromantes dehaani (67.57%) and Helice tientsinensis (17.28%). The overall size frequency distribution was bimodal for C. dehaani and unimodal for H. tientsinensis. The estimated annual production of C. dehaani and H. tientsinensis was 43.981 g and 4.247 g ash-free dry weight (AFDM) m−2 year−1, respectively. The annual P/B ratio was 1.22 for C. dehaani and 0.75 for H. tientsinensis. In total, the annual secondary production of all crabs at the Dongtan salt marshes was estimated to be approximately 49.405 g AFDM m−2 year−1. The consumption of vascular plants by C. dehaani and H. tientsinensis was 480.831 and 42.640 g AFDM m−2 year−1. About 20.4% of plant primary production was ingested by these two herbivorous crabs. The production of C. dehaani was high compared with those reported from other coastal wetland ecosystems. This suggests that the ecological functions of C. dehaani at the Yangtze River estuary salt marshes need more quantitative studies to be fully described.