Results of a greenhouse experiment showed that water level gradient had significant effects on morphological traits, clonal characteristics, biomass accumulation and biomass allocation patterns of Spartina anglica. The effects of water level gradient on the morphological traits and biomass allocation of S. anglica were studied in the glasshouse with populations coming from the Xinyang Harbor in Jiangsu Province, China. Five treatments were set up with water levels from submergence to 1/2 of the plant height, 1/3 of plant height, and water level at substrate level, -5 cm and -10 cm below the substrate surface. With the submergence of 1/3 of plant height and water at substrate level treatments, plant height, number of ramets, number of rhizome nodes, total rhizome length, spacer length and biomass production were significantly higher than those of -10 cm and 1/2 plant height treatments (p < 0.05); and these parameters with -5 cm treatment were not significantly different from those of the submergence of 1/3 of plant height or water at substrate level treatments (p > 0.05). With the -10 cm treatment, biomass allocations to above\|ground and rhizomes were significantly lower than other treatments. The results indicate that S. anglica populations grow better with water submergence from substrate surface to 1/3 of plant height. The results also indicate that change in hypsography and subsequently degree of inundation for S. anglica may explain the decline of its populations over its range.