Dam is considered as a serious threatening factor for migratory fishes on a global scale. The quantitative mechanism of dam’s impact on anadromous fish, including the Chinese stur-geon in the Yangtze River, is an unsolved issue. Here we propose an essential theory covering four theoretical models of fish migration, breeding and population size, to reveal the relationship between dams and fish, and discover novel insights in migration behaviors, gonadal degeneration, and population dynamics of the Chinese sturgeon. We obtain the temporal-spatial distributions, gonadal development and population size annual variations of the Chinses sturgeon and then evaluate impacts of the Yangtze dams. We show that since 1981 the Gezhouba Dam has reduced the migration distance by 1,175 km and compelled the Chinese sturgeon to spawn in a narrow reach downstream from the Gezhouba Dam, resulting in gonadal development delayed by 37 days, the effective breeding population size and the environmental capacity of new spawning ground respectively reduced to 24.1% and 6.5% of the original. Even worse, the subsequent dams particularly the Three Gorges Dam and Xiluodu Dam have further reduced the effective breeding quantity to merely remaining 0~4.5% by elevating water temperature to inhibit the breeding activity during the spawning season. We highlight that the declines of environmental carrying capacity and the effective breeding quantity are two crucial reasons for the prolonged risk of the Chinese sturgeon existence, and the cumulative effect of cascade dams has led to an ongoing decline in the adult abundances in the Yangtze River and the sea, from 32,260 individuals (1,727 of the Yangtze) before 1981 to 6,000 (190) in 2010 and to 2,569 (156) in 2015. If no viable remedial measure, the wild Chinese sturgeon will become extinct in 10~20 years. We suggest the joint operation of upstream cascade reservoirs on the Yangtze River to mitigate the dams’ effect and to res-cue the natural population.