This study investigated the feeding intensity and growth response of different-sized grass carp (Ctenopharyngodon idellus) to Eichhornia crassipes, aiming to provide scientific basis for controlling the overgrowth of E. crassipes, maintaining aquatic ecological balance and realizing the resource utilization. Ten size groups of grass carp with body weight ranging from 50 to 500 g/ind were selected. Thirty individuals from each group were reared in net cages and fed exclusively with E. crassipes for a 120-day culture period. The growth indices and survival rate of grass carp, as well as the nutritional composition of E. crassipes, were monitored to analyze the feeding and growth characteristics of grass carp across different body sizes. The water quality of the experimental water body complied with fishery aquaculture standards. E. crassipes contained 17 amino acids, among which essential amino acids accounted for 30.86% of the total amino acid content, and 15 fatty acids, with polyunsaturated fatty acids accounting for 50.59% of the total. By contrast, the crude protein and crude lipid contents of E. crassipes were relatively low, at 11.66% and 0.91%, respectively. The body weight of grass carp in all groups increased significantly, and the total length increased extremely significantly. For grass carp with body size above 250 g/ind, the net weight gain and feed intake increased rapidly, with the maximum weight gain rate reaching 167.11%, and the feed conversion ratio decreased with the increase of body size. The condition factor of grass carp fed only with E. crassipes was lower than that of grass carp fed with formulated feed, presenting a slender body shape, and the survival rate of small-sized grass carp was relatively low. The body size of grass carp is positively correlated with the feed intake of E. crassipes. Grass carp with body size above 200 g/ind, have significantly enhanced feeding and utilization capacity for E. crassipes. Grass carp with appropriate body sizes can balance the biomass of E. crassipes and the water purification function of water bodies. E. crassipes is suitable to be used as supplementary feed rather than the sole nutrient source for grass carp.