Abstract: The construction of age-friendly and healthy cities needs to clarify the benefits of blue-green space on the elderly’s mental health. In light of the one-dimensionality of existing literature in terms of indicator selection and spatial scale, this research systematically compares the differences between the qualitative dimension (internal environment characteristics, and landscape pattern index) and the quantitative dimension (visibility and availability) of blue-green space in their effects on the elderly’s mental health, explores the intrinsic causes of such differences by examining the mediating effects of activity capability, environmental perception and social cohesion, and verifies the robustness of conclusions at multiple spatial scales. Research results show that the visibility of green space can bring more benefits to the elderly’s mental health than the availability thereof; diverse mediating pathways can significantly strengthen the association between the qualitative dimension of blue-green space and the elderly’s mental health; the association between blue-green space and the elderly’s mental health and relevant mediating effects show regular variation characteristics under the multi-spatial circles. Based on the conclusions above, the research proposes suggestions for planning urban blue-green space oriented at the elderly’s mental health.