The present study was carried out to investigate the growth characteristics of different ryegrass (Lolium perenne L.) cultivars and their ability to remediate eutrophic water using floating plant-bed technology. Greenhouse and lake experiments were conducted to evaluate the grass genotypes for water remediation. Twelve cultivars of ryegrass including Grazer, Secale Cerale, Energa, Rustmaster, AngusI, Abundant, AngusII, Jivet, Gulf, Surrey, Major and Barwoltra were grown in the floating plant-bed system. The plant biomass, plant NP (nitrogen and phosphorus) accumulations and the water purification capacity of selected grasses were significantly different (P < 0.05). Abundant, AngusII and Major showed most efficient purification capacity of eutrophic water. In a greenhouse, after 26 days of growth, the eutrophic water was purified to various extents by the different ryegrass cultivars. Nitrogen removal efficiency varied from 52.20% to 73.82% and phosphorus removal efficiency ranged from 75.12% to 84.77%. In a lake experiment at Huajiachi pond, after 162 days of growth, the plant shoot biomass increased from 321.5 to 922.8 g/m2 dry weight basis, shoot NP accumulation ranged from 61.5 to 168.2 mg m−2 d−1 and 11.9 to 47.2 mg m−2 d−1 respectively. NP accumulation rate by the various cultivars of ryegrass was highly positively correlated to their biomass production and water purification capacity. Thus, plant biomass could be used as an indicator for assessing purification capacity of a ryegrass cultivar.