Abstract: The affordance theory describes the psychological essence of children's environmental perception and their behavioral response to specific environment in the way of function, reflecting strong humanistic care. Through literature review, this paper first clarifies that the affordance theory has become an important basis for the research of children's environmental perception because it can directly describe children's environment-behavior relationship. By different dimensions, levels and attributes of affordance, the various activities opportunities provided by open spaces for children can be described in detail. As a medium concept to connect environment with human perception and behavior, affordance actually describes conditions of behavior occurrence and explains how physical, social and cultural and individual factors affect children's realization of affordances and even children's development. Measuring and analyzing affordance constitutes the basic research method of child-friendly open spaces based on affordance theory at present. The research of child-friendly open spaces based on affordance theory also goes beyond the space design decision and efficiency evaluation and further provides the possibility of establishing a research and practice framework oriented by evidence-based design.