Abstract: 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.

Key words: Sedentary lifestyle, Television, Body mass index, Child behavior, Family