Preserving urban biodiversity is widely recognized as a vital nature-based solution that not only enhances ecological integrity but also promotes social welfare. However, the specific mechanisms through which urban biodiversity impacts public mental health remain underexplored. Particularly in densely populated metropolitan areas, urban parks not only serve as essential recreational spaces for residents but also play an irreplaceable role in maintaining biodiversity, mitigating environmental stress, and enhancing human well-being. This study focused on five typical green spaces within four urban parks in Shanghai, examining visitors' perceptions of biodiversity (plants and birds) within these diverse environments. Structural Equation Modeling (SEM) was employed to analyze the dynamic relationships and potential causal pathways among perceived biodiversity, perceived naturalness, restorative benefits, and emotional states (positive and negative). The findings revealed several key insights: 1) Open green spaces with water bodies (OGW) exhibited the best performance in terms of perceived bird diversity, positive emotions, and restorative benefits. In contrast, enclosed green spaces (EGS) and semi-open green spaces (SOGS) showed advantages in perceived plant diversity. 2) SEM analysis indicated that while perceived biodiversity did not significantly influence perceived naturalness (P>0.05), both perceived biodiversity and naturalness significantly fostered restorative benefits, improved positive emotions, and reduced negative emotions (P<0.05). 3) Mediation tests demonstrated that restorative benefits played a crucial mediating role between perceived biodiversity and emotional states. Notably, perceived bird diversity (P<0.001) exerted more significant direct and indirect impact on emotional states compared to perceived plant diversity (P<0.01). Based on these results, several measures were proposed to enhance visitors' restorative benefits and emotional well-being. For instance, urban park designs should prioritize the incorporation of native flora and adaptive plant species to attract diverse bird populations. Creating varied microhabitats can promote natural biodiversity growth while fostering a rich soundscape, which in turn enhances visitors' positive emotions and restorative experiences. Additionally, incorporating water features, such as small lakes, fountains, or ponds, not only provides crucial resources for wildlife but also enhances the aesthetic and tranquility of the landscape. Finally, ensuring the safety and accessibility of different green spaces through well-designed paths and recreational facilities can increase their attractiveness and encourage frequent use by diverse visitor groups. The study underscores the importance of integrating biodiversity into urban planning and park management practices, advocating for a holistic approach to advance urban environmental and public health strategies. By optimizing green space design, particularly in high-density urban areas, cities can simultaneously improve residents' mental well-being and promote the synergistic development of urban ecological and social benefits. These findings provide a theoretical foundation for the planning and design of restorative urban green spaces aimed at maximizing biodiversity and human well-being.