Objective: To explore the role of outdoor activity in the relationship between infancy responsive caregiving trajectories and early childhood development, so as to provide a theoretical basis for the promotion of early child development. Methods: The study participants were drawn from the Shanghai Maternal-Child Pairs Cohort and 4 723 mother-child pairs who completed responsive caregiving questionnaires at 2, 6 and 12 months old were included. Questionnaires were used to assess children s responsive caregiving and average daily hours of outdoor activity at 2 years of age. The Age-Stage Questionnaire, Third Edition (ASQ-3) was used to evaluate children s development problems at 2-5 years old. Group based trajectory model was applied to fit infancy responsive caregiving trajectory. Modified Poisson regression was used to analyze associations between different responsive caregiving trajectory groups and child development, and moderating effects were tested for hours of outdoor activity. Results: Infancy responsive caregiving trajectories were categorized into general group ( n =3 871), declining group( n =160), and fluctuating group( n =646). After adjusting for confounding factors, such as parents educational level, annual household income, maternal progestation body mass index,maternal tobacco exposure during pregnancy,maternal anxiety and depression during pregnancy, maternal age at delivery,maternal gestational age,maternal mode of delivery, children s gender,children s birth weight, and duration of breastfeeding, the results of modified Poission regression analysis showed that compared with the general group, children at the age of 2 in declining and fluctuating group had increased risks of suspected developmental delays in communication, gross motor, fine motor, problem solving, and personal-social scales ( OR =1.41,1.31,1.35,1.23,1.21；1.07,1.08,1.08,1.09,1.06);but children only had increased risk of suspected developmental delays in communication of declining group ( OR =1.08), personal-social scales of fluctuating group ( OR =1.06) at 3-5 years of age ( P <0.05). At lower levels of outdoor activity, children in fluctuating group had reduced scores in communication ( β =-1.41), fine motor ( β =-2.34), problem solving ( β =-1.11) and personal-social scales ( β =-1.99) as compared to general group; and children in declining group had reduced scores in gross motor ( β =-4.78)( P <0.05). While at higher levels of outdoor activity, no differences were found between children in fluctuating, declining groups and those in general group in scores of different scales ( P >0.05). Conclusion Prolonged outdoor activity attenuates the adverse effects of declining and fluctuating trajectories of infancy responsive caregiving on early childhood development.