Abstract: Freshwater mussels are one of the most widespread and imperiled groups of animals in the world, which play a major part in the functioning of many freshwater ecosystems, however, their populations declined continually during the 20th century. Unio douglasiae, one important specie of Unionidae, is widespread in lakes, rivers and impoundments, but little is know about its population structure and reproductive traits. Investigations on the population structure and reproductive traits of Unio douglasiae were carried out in Qinglan Lake from July, 2008 to July, 2009. From the view of population structure, the population tended to be stable and shell length of individuals was mainly from 40 mm to 65 mm in the population. The maximum length and the minimum length of individuals were 83.6 mm and 20.3 mm in the specimens. The sex ratio of natural population was found 1∶1.20, the sex ratio of each month was in close agreement with the expected value of 1∶1 by χ2 test. The minimum mature female with a shell length of 27.6 mm and weight of 2.62 g, and which of the male with a shell length of 27.9 mm and the weight of 2.21 g were found. The minimum pregnancy individual was found with a shell length of 29.8 mm and weight of 2.98 g in examined specimens. We concluded that the female and male individuals matured at the age of 1 year. Taking the developmental characteristics gametes in follicles and features of follicles into account, the annual cycle of maturation for both male and female could be divided into the following three periods: (1) growth and maturation period, October to January of next year, (2) spawning period, February to June, (3) recovery period, August to September. The female in maturation could spawn several rounds in one reproduction cycle. Two pieces of outer gills of the female were used as marsupium. Embryo developed in the conglutinates. In the development process of embryo, the colour of marsupium changed from ivory white to light red, then became cardinal red after matured. Mature glochidia on a mucous thread released from the water column. After stimulated by stressed condition, pregnancy mussels would release the superconglutinate. The breeding season lasted from February to July. The breeding peak period occurred from April to June, the pregnancy rates were 80.43%, 84.21% and 78.05% respectively. The average fecundity reached about (183.72±6.43)×103 embryo or glochidia. The maximum fecundity was 344.64×103 embryo or glochidia and the length of individual was 54.5 mm. The minimum fecundity was 49.95×103 embryo or glochidia and the length of individual was 29.8 mm. Fecundity was related significantly to both length and weight. This study may provide more information related to basic biology and guidance for conservation of freshwater mussels and science-based management of rivers and lakes.