Little is known about what spatial features of park that old people as most importance for encouraging them to visit and be active and social in parks. This paper examined the relative significance of park and explored different spatial factors in connection with flexibility and diversity that effect social interaction among different aged groups in the context of China. Social interaction is significant and beneficial for older people’s mental healthy, which also contributes to relatively main part for maintaining social sustainability. Utilizing extensive empirical field work to do observation and ethnographic interviews on where and how social activity occur among older people, and based on amount of literature review explored relationship in-between social interaction and essential effects in terms of flexibility and diversity, to examine and retest how they enact in park. This paper used micro-design analysis approach reveal social interaction more to do with ability of square of park, which strengthen social interaction via diverse functional ‘in-betweenness’ (emphasizing on children-playing function), regularity of props and edge that enhanced with active views within ‘positive’ space, which extends concept of flexibility. Moreover, adjacent diverse function not only extends concept of diversity, also being conductive with flexibility, to create more social interaction. Besides, seasonal changeable spatial feature as unfamiliarity challenged related theory that can create more social opportunities among old people within familiar space, and noticed timeliness that as novelty and complexity to renew quality of flexibility. Likewise, social condition in terms of congestion and events can also be confirmed. Ultimately, the findings bring optimal suggestions to practice and offer new insights to design more age-friendly public space in city and contribute to social sustainability.