The mudskipper Periophthalmodon septemradiatus is an amphibious fish and widely distributed in mudflats and the mangrove swamps in the regions of Asia. In the Mekong Delta, P. septemradiatus lives from the estuary to the upstream and is a potential aquarium pet. However, the fish population size has been reduced due to urbanization and environmental pollution. Knowing on alimentary tract morphology and Clark index; and the variation of Clark value with season, fish size, site, and month have been unknown. Data analysis of 1,504 fish (930 males and 574 females) caught from the Hau River for a year-round survey, from August 2017 to July 2018 showed that this fish species is a carnivorous species based on its alimentary tract morphology. As Clark value of this species did not vary with season and site, suggesting that the energy accumulated ability of P. septemradiatus was not influenced by seasonal and spatial variation. However, the mudskipper’s energy accumulated ability was regulated by fish size and month since Clark value fluctuated with size and month. These results were used for the understanding of fish adaption to the study region.