目的：研究儿童视屏行为和家庭相关因素与儿童体重指数（body mass index，BMI)的关系，以及家庭相关因素如何通过影响儿童视屏行为而作用于儿童BMI。方法：选择北京市某城区12所小学7~11岁共1 846名学生作为研究对象，对学生进行身高、体重测量，计算BMI。通过问卷调查学生的视屏行为时间及其相关的家庭因素。儿童的视屏行为时间是学生回忆过去7 d平均每天看电视/视频的时间和使用电脑/游戏机/iPad的时间。调查可能与儿童视屏行为相关的家庭因素，包括父母的文化程度、职业、视屏行为时间、父母是否告诉儿童视屏行为的不良影响、父母限制儿童视屏行为时间。由父母文化程度和职业计算得到家庭社会经济地位评分。结果：儿童视屏行为时间的中位数为1 h/d，四分位间距为1 h/d。父母限制视屏行为时间<120 min/d者与父母限制视屏行为时间≥120 min/d者相比，男生BMI降低了1.63 kg/m²（P<0.001），女生降低了0.85 kg/m²（P=0.004）。家庭经济地位评分、父母视屏行为时间、父母是否告诉儿童视屏行为的不良影响与儿童的BMI无关。男、女生视屏行为时间在父母限制儿童视屏行为时间和男、女生BMI关系的中介效应值分别为-0.222 kg/m²（95%CI：-0.432，-0.095）、-0.187 kg/m²（95%CI：-0.507，-0.049），中介效应占总效应的比例分别为13.67%、22.11%。结论：父母限制儿童视屏行为时间通过影响儿童视屏行为时间而作用于儿童BMI。家长对儿童的教育对儿童BMI的作用不如家长对儿童行为的监督。所以，在今后儿童肥胖的预防和控制工作中，家长参与监督儿童的行为是必不可少的。

Objective：To explore the relations among screen-based sedentary behaviors (SSB), family factors and body mass index (BMI) of children, and to study how family factors have effect on BMI through influencing SSB. Methods: A total of 1 846 students aged 7-11 years from 12 primary schools in one district of Beijing were included. Their body weight and height  were measured to calculate the BMI. The time of SSB and family factors were investigated by using questionnaires. The time of SSB was the total time of watching TV and videos, playing computer games and iPad each day during the past 7 days recalled by children. The family factors included the parents’ education, occupation, the parents’time of SSB, whether the parents told their child the harm of SSB, the parents’time limit for the children’s SSB. The parents’education and occupation were used for calculating the family socioeconomic score. Results: The median time of SSB for children was 1 hour/day, and the interquartile range was 1 hour/day. The BMI of the children with the parents’time limit for the children’s SSB less than 120 min/day were smaller than the children with the parents’time limit not less than 120 min/day, in both the boys(1.63 kg/m²，P<0.001) and the girls (0.85 kg/m², P=0.004). The family socioeconomic score, the parents’SSB time, whether the parents told their children the harm of SSB were not related to the children’s BMI . The mediation effects of SSB time for children on the association between the parents’time limit for the children’s SSB and  BMI  were-0.222 kg/m²(95%CI:-0.432, -0.095) for boys and -0.187 kg/m² (95%CI: -0.507, -0.049) for girls , which accounted for 13.67% of the total effects for boys and 22.11% for girls.  Conclusion:The parents’ time limit for the children’s SSB has effect on their BMI  through influencing their SSB time. Parents’ supervision on the behaviors of children produces larger benefit for BMI than health education conveyed by parents. Therefore, parents’ participation in supervising the behaviors of children are indispensable for preventing and controlling childhood obesity.