Understanding how environmental changes affect life-history traits is important for predicting population dynamics of a species. As one of the most pervasive forms of environmental changes in freshwater systems, eutrophication can change life-history traits of freshwater animals through density-dependent/independent processes. However, empirical evidence remains scarce regarding the mechanisms by which eutrophication influences life-history traits. We conducted a field study linking eutrophication, life-history traits and population density in a population of Margarya melanioides in Lake Dianchi, the largest lake of the Yun-Gui plateau in China and the most eutrophicated by humans, over 14 months in 2009-2010. We estimated key life-history traits (growth and reproduction) of M. melanioides in 2009-2010. Then, by comparing some life-history traits with those recorded in 1931-1933 (when the extent of eutrophication was comparatively low), we found that this snail had increased its fecundity (from 1.07 to 4.24 offspring/brood-bearing female) and the proportion of females with broods (23.6 to 77.4%), decreased its body size (54.2 to 47.6 mm) and the threshold size for reproduction (46 to 35 mm), and distorted its female/male sex ratio (1.6:1 to 0.97:1). These findings suggest that M. melanioides has adapted some life-history traits in response to intensified eutrophication. A structural equation model indicated that eutrophication influenced the reproduction of this snail through two related but opposing pathways: primarily by decreasing its population size to reduce its reproduction and secondarily by directly promoting its reproduction. Our results not only support the significance of eutrophication in determination of life-history traits of freshwater animals, but also suggest that density-dependent effects on reproduction may be stronger than direct environmental influences when population size is small.