Background: Urban green open space is a valuable resource for physical activities of urban inhabitants and has the potential to reduce chronic illness and improve health. Research on the relationships between green open space and physical activity is incomplete and limited in China. Thus, the study examines how the urban green open space contributes to physical activity.

Methods: A questionnaire was designed based on the social ecology theory to investigate the physical activity of 513 residents in urban green open space. We use the time and frequency of residents exercising in urban green space to measure physical activity, and use the factor analysis to synthesize a large number of original factors (i.e., infrastructure, safety, accessibility, landscape quality, and space environment) into relatively few composite indicators. Based on the collected data of the cross-sectional population, the Order Probit regression model was constructed to analyze how urban green open space affects the residents' physical activity from the perspective of social ecology.

Results: 1 in community factors: accessibility is significantly positive correlation with residents' physical activity, and there is no significant correlation between safety and physical activity; 2in natural factors: space environment and landscape quality are not significantly correlated with residents' physical activity; 3 in built environmental factors: infrastructures, the area of green space, the size of open space, and entertainment facilities are significantly correlated to residents' activity. Basketball courts, volleyball courts, swimming pools, and sports equipment will promote physical activity; 4 apart from the attributes of green open space, other factors are significantly correlated to physical activity in the green open space, e.g. having a companion.

Conclusions: Urban green open space plays an important role in promoting physical activity especially among the women and the old, and improving the attributes (such as accessibility, infrastructures, the area of green space, the size of open space and entertainment facilities) of the urban green open space and trying to set up group sports proper to play with companion (like "square dancing" and "Tai Chi") can promote Chinese residents' physical activity so as to improve public health. The results are significant to facilitate environment health.

The authors declare that they have no competing interests.

• 71763001/National Natural Science Foundation of China