Background: It is known that short sleep duration adversely affects children's behavior and physical development. This study aimed to investigate the status of sleep duration in 3-14-year-old children in Beijing and explore the related factors of sleep loss with them. Methods: In this study, a cross-sectional study of random stratified cluster sampling was conducted on 3-14-year-old children and adolescents in Beijing. According to the proportion of children in each district and school, the final cohort included a total of 11 kindergartens, 7 primary schools, and 8 junior high schools from 7 districts of Beijing. Children of sampled classes were included, and their parents were invited to fill a series of questionnaires including the simplified Chinese version of Pediatric Sleep Questionnaire, Sleep Questionnaire Scale, and Hong Kong-Children Sleep Questionnaire about the performance of the last 6 months. Results: Out of the total 11,420 questionnaires, 9198 questionnaires were valid and effective with the response rate of 80.54%. The age of the investigated children was 8.8 +/- 3.8 years, including 4736 males and 4462 females. The daily sleep duration of children in Beijing was 9.7 +/- 0.7 h. The prevalence of sleep loss (<9 h/day) of children in Beijing was 11.8%. The daily sleep duration of children aged <6, 6 <= age <11, and >= 11 years was 9.7 +/- 0.6 h, 9.6 +/- 0.6 h, and 9.5 +/- 0.8 h, respectively. The sleep duration reduced significantly in children aged >= 11 years as compared to younger children in Beijing which was mainly contributed by the variation tendency of sleep duration on weekdays. The multivariate logistic regression analysis identified factors associated with sleep loss (P < 0.05): male (odds ratio [OR] = 1.32, 95% confidence interval [CI]: 1.15-1.51), age >= 11 years (OR = 2.37, 95% CI: 1.92-2.93), overweight (OR = 1.34, 95% CI: 1.17-1.54), family history of snoring (OR = 1.35, 95% CI: 1.13-1.61) and activities before bedtime with watching TV (OR = 1.24,95% CI: 1.08-1.43), sports (OR = 1.22, 95% CI: 1.01-1.48), playing cellphone (OR = 1.91, 95% CI: 1.31-2.73) and surfing the Internet (OR = 1.27, 95% CI: 1.06-1.52) and among them age >= 11 years and playing cellphone before bedtime had greater impact on children's short sleep duration than that of other factors. Conclusions: Sleep loss was common among 3-14-year-old children in Beijing. Sleep duration decreased with age, especially among children over 11 years old. Factors associated with sleep loss covered sociodemographic characteristics, family sleep habits and routine activities before bedtime, and among those variables, age >= 11 years and playing with cellphones before bedtime had a greater impact on sleep duration, indicating that existing sleep loss in 3-14-year-old children could be, at least partly, improved by paying more attention to children aged of 11 years or entering Grade 5 and Grade 6 and to children with a family history of snoring; by reducing the use of electronic products before bedtime, especially cellphones; by managing weight and keeping fit; and by improving the bedtime routine.