Open space can be stress alleviating and healing through the appropriate enclosure. Based on theories about environmental preferences and empirical studies on spatial enclosure, this study examined the relationship between environmental preference indicators and enclosure of urban public open spaces in Hong Kong. The preference indicators including four perceptual attributes and feedbacks on six feelings were evaluated together with the spatial enclosure of 178 subjects based on images of 26 single spaces selected in Hong Kong. All of the four perceptual attributes and five out of six feelings showed significant differences between different spatial enclosures. The results showed that subjects from Hong Kong tended to prefer more open spaces to enclosed ones. Through further interpretations, paths, visual connection with adjacent spaces, and a clear and simple spatial structure of enclosure seemed to influence subjects’ feelings and their preferences towards certain spaces. When spatial enclosure is somehow ambiguous, the space could still be preferred as long as the relationship between the single space and its adjacent spaces is clear. Design implications are then discussed for public space in high-rise high-density urban contexts.