Within urban environment with high-rise buildings and structures, walls represent the most common vertical spaces. Conventionally, such spaces are viewed as abiotic areas, where spontaneous flora is neglected. Through investigations in a typical mountainous city Chongqing, this study concerns the spontaneous species diversity on walls and the influences of wall factors and the adjacent environment. A total of 239 vascular plant species belonging to 172 genera and 75 families were found; 90% of the species are indigenous. More fern species inhabit walls, compared with xerophytes that dominate general urban environment. Variation partitioning indicated that wall attributes played a more important role in explaining the total variation in wall vegetation composition and structure than did the adjacent environment. Given that there are limited possibilities to extend more green space in urban land environment, we support a "let Nature take its course" approach to improve urban biodiversity, where vertical urban space with spontaneous flora can act as a valuable complement to biodiversity and ecosystem services in dense urban environment. This study should raise urban designers' and ecosystem managers' awareness of the possibilities of this type of informal, unconventional habitat as a "supplement" for urban greening and landscaping.