Scaly-sided Merganser (Mergus squamatus) is a typical indicator species of forest stream ecosystem, and rarely reported in man-made bodies of water. Wintering population of M. squamatus had been continuously monitored in the Huidong baipenzhu reservoir in Guangdong province from Sep. 2007 to Mar. 2013. The results are as follows: 1) First M. squamatus arrived at Baipenzhu reservoir at the end of September and left in the March of the next year, and stayed here for three months to six months. 2) The largest population of M. squamatus remained stable from 2007 to 2009 (31 in each year), but reduced year after year from 2009 to 2013 (only 12 in 2013). 3) M. squamatus were mainly distributed nearby the DaLuoDong in reservoir, and their home range were between 100.01 hm2 and 180.9 hm2. The man-made water body with large area, cleaned water and less-disturbed by the outside world in this area are possible reasons for M. squamatus wintering here. The studies indicated that the suitable wintering environment and the effective management measures of local natural reserve could contributed to the species wintering here. We propose to classify DaLuoDong into core zone of the Lianhuashan-Baipenzhu Provincial Nature Reserve in order to further strengthen the protection of the M. Squamatus.