Abstract: Adequate outdoor activities are of great significance to the healthy development of children. However, with the continuous increase of urban population and building density under the background of rapid urbanization, the outdoor space for urban children urgently needs to be expanded. Although rooftop space of public buildings is a huge stock of urban resources, currently the consideration of children's benefits is very limited. By analyzing the carrier and expansion path of today's urban children's outdoor activity space, this study point out the advantages and significances of using public building rooftops as children's outdoor playground. From the Child-friendly perspective, combined with prototype extraction and case analysis, this paper discussed the environmental characteristics of public building rooftop outdoor activity space from four aspects:spatial form, spatial interface, traffic connection and landscape theme. Finally, some suggestions were put forward such as attaching importance to children's rights and interests, supplying a diversified supplements to the outdoor activity space of urban children, and improving the quality of rooftop spaces, so as to provide useful ideas for expanding urban outdoor space for children and promoting the construction of child-friendly cities.