To analyze the biological and dietary characteristics of Acanthogobius ommaturus, a total of 186 individuals were collected in the salt marshes in the Yangtze Estuary from May 2015 to April 2016, with the highest abundance collected occurring in the summer (June, July, and August). About 96% of the total specimens were young fish with an average body length of 109 mm and an average feeding level of 1.5. The prey items included 11 groups and 41 species dominated by shrimps (Index of Relativity Important, IRI (%)=69.05), fishes (IRI (%)=17.31), and crabs (IRI (%)=11.51). The dominant species of shrimp prey items were Exopalaemon annandalei and Macrobrachium nipponense, and of fish, Periophthalmus magnuspinnatus and Boleophthalmus valenciennes. The mean stomach fullness index P (%) varied, and increased slowly from June-November. The proportion of fish prey items increased with increasing length, whereas the biomass proportion of fish prey items in adult species was above 65% and increased with increasing body length Food composition of Acanthogobius ommaturus was analyzed for differences in the day and night from June-August. The weight proportion of fish prey items during the day was 8 times higher than at night (P < 0.01), whereas for shrimp, weight proportions were 2 times higher at night than in the day (P < 0.01). There was no significant difference for crabs between the day and night. Three body length groups (41-60 mm, 61-180 mm, and 181-200 mm) were identified for Acanthogobius ommaturus individuals at the 10% similarity level analyzed by CLUSTER. For the trophic niche of Acanthogobius ommaturus in each size class, the trophic niche in the 41-60 mm group was 0.86. By contrast, the trophic niche increased in the 61-120 mm group, and decreased in the 121-160 mm group. Therefore, the rich food environment in the salt marsh supplies a good feeding place for Acanthogobius ommaturus, and the dense plants can supply the best shelter for young fishes. These can help young fishes grow to adults in the autumn, and can then move to deeper waters for the winter. In conclusion, our study preliminarily indicates that the salt marsh plays crucial roles in the development of Acanthogobius ommaturus.