Biodiversity is of great significance to ensure human society’s sustainable development and human well-being. However, it is tough to embody the value of biodiversity and recreation of urban green spaces in practice. The main reason is the lack of people’s knowledge of biodiversity perception and preference. Taking the Small West Lake Wetland Park in Xianyang City, Shaanxi Province, as an example, we evaluated the plant diversity of the Small West Lake based on field surveys and a plant diversity evaluation system. The visitor-employed photography (VEP) was used to obtain the respondents’ perception of the Small West Lake plant diversity and recreational preference. Finally, the relationship between actual plant diversity, human perception, and recreational choice was analyzed. In the five types of green spaces in Small West Lake, the plant diversity of green spaces with stratified structures was the highest. People could correctly identify the plant diversity of different biotopes and preferred water environments and green spaces with stratified structures with rich biodiversity. Overall, the respondents believed that the plant diversity of Small West Lake was high, and they considered management as the most critical measure for plant diversity conservation in urban green spaces. The results can provide a theoretical basis and practical methods for landscape planning and design that balance plant diversity conservation of urban green spaces and residents’ outdoor recreation needs.