We made population census on the Mute Swan (Cygnus olor) during the breeding season and pre-migration period in the Wuliangsuhai Lake from 2014 to 2017 by line transects and point counts methods (Fig. 1), compared with data and records in past decades, and analyzed the causes of the changes of the populations. The breeding adults in springs of 2015 to 2017 remained basically stable, with the number of 84, 92, and 80 individuals, however, the autumn population of 2014 to 2017 declined year by year, with the number of 411, 302, 281, and 153 individuals respectively (Table 1), possibly because that the reduction of suitable breeding and feeding habitat, predation, egg collecting by local people and poisoning factors have affected the survival of sub-adults and juveniles in recent years. From 1996 to 2004, the number of Mute Swan populations increased as the establishment of the nature reserves, with hunting and egg-collecting being forbidden. From 2005 to 2013, the number of populations declined due to drought and water shortage, development and utilization of waters, expansion of reed (Phragmites australis) and broad leaf cattail (Typha latifolia), deterioration of water quality, and increased eutrophication. The research showed that the Wuliangsuhai Lake has been overexploited and utilized, and frequent human interference affecting the breeding of Mute Swan in recent years. Scientific management on the habitats and more strictly control of egg collecting and poisoning are essential to the survival and population growth of Mute Swan.