Climate change and the rapid loss of biodiversity pose threats to human survival and development. However, there remains a limited understanding of the relationship between biodiversity and human health, especially in the context of developing countries. This study pursues two objectives: (1) to explore the relationship between biodiversity and human health; and (2) to investigate how this relationship varies across different economic regions. Using a cross-sectional design, we analyzed data from 31 provinces, autonomous regions and municipalities across China. The dataset encompassed information on biodiversity (animals, plants, and fungi), subjective health, and mental health. We employed a two-stage least squares analysis, designating the proportion of part-time rangers and the completion of investments in wild fauna and flora per unit area as instrumental variables. Subsequently, we examined the moderating effect of the average Per Capita Disposable Income on the relationship between biodiversity and mental health. Additionally, we conducted grouped regression analyses for three regions with varying levels of economic development. The regression model incorporated macroecological, macroeconomic, and sociodemographic data as control variables. The results indicated a correlation between bird, plant species richness, biological richness (the total number of plants, animals, and fungi), and mental health. However, these biodiversity measures were significantly associated with poor subjective health. Additionally, the contribution of biological richness to mental health declines as the average Per Capita Disposable Income rises. The relationship between biodiversity and mental health was significantly negative in regions with low economic development, significantly positive in medium economic development regions, and not significant in high economic development regions. The analysis indicates that as the economy grows, there may exist a lower and upper threshold at which biodiversity exerts a positive impact on mental health. These thresholds merit further investigation. This observed gap might be attributed to the diminishing natural experiences in regions undergoing rapid economic development. The findings of this study offer insights that could guide policy decisions in developing countries, balancing biodiversity preservation, economic growth, and public health.