From January to March 2016, and in November 2016, we examined the effects of sex and ambient temperature on how time was budgeted, and activity rhythms were established in wintering Mandarin Ducks (Aix galericulata). This was done using instantaneous scan sampling in the Poyang Lake watershed in the Jiangxi province, China. The results showed that the Mandarin Ducks’ predominant behaviors were resting (36.56%), swimming (21.75%), foraging (20.88%), and maintenance (15.58%), indicating that during the winter, Mandarin Ducks spend most of their time accumulating energy. Mandarin Ducks reduced their energy consumption by increasing time spent on resting behavior. Females spent more time resting than the males, and less time on maintenance and vigilance. No significant difference was found in the allocation of time to different behaviors under different ambient temperatures. Foraging behaviors peaked in the morning (10:00–10:59) and afternoon (15:00–17:30), which is like the foraging strategies of most Anseriformes. Mandarin Ducks increased their foraging time in the mornings and afternoons to get maximal energy for the long cold nights. Occurrences of foraging and swimming behaviors showed similar patterns, because foraging was often accompanied by swimming to find suitable foraging sites. The pattern of resting behaviors was opposite to that of swimming. Sex had little effect on daily activity rhythms, mainly because both were involved in the same tasks to support similar requirements.