The increase in urban population and the expansion of urbanization have had various physical, social, environmental, and economic consequences for cities and their citizens, especially psychological consequences; On the other hand, it can be said that contact with nature in today's cities in the form of urban green spaces in various scales plays a significant role in reducing mental and psychological problems. Hence, urban green infrastructure planning, considering spatial/physical qualities to improve citizens' mental health, can play a role in transforming urban spaces into supportive environments. Green infrastructures have become influential components not only in their content, but also in their impact on their surrounding areas' physical, environmental, and social values in recent decades. The main aim of this research is to test and analyze the relationship between urban green infrastructure and citizens' mental health in the surrounding area of Park-e-Mellat to enjoy the capacities of such spaces in promoting urban health. In this article, we have attempted to investigate the relationship between the surrounding area of the urban park and its effect on mental health through document and field methods (interview and questionnaire). Thematic analysis using semi-structured interviews with 14 space users was performed using MAXQDA software. In the second stage, using the fuzzy Delphi technique, the final results were obtained with 10 experts in two rounds of the questionnaire. The research method is based on an interpretive paradigm, with a mixed approach in the first part of qualitative thematic analysis and the second part of fuzzy Delphi. The findings, in the form of four main themes: the quality of the physical environment, mental health, physical health, and social health, defined ten qualities for green infrastructure planning to improve citizens' mental health.