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Urban public green spaces are widely recognized for having positive effects on mental health, yet existing research shows imbalances in subjects and methodologies. Most studies examine healthy adults and self-reported indicators, giving limited attention to vulnerable populations; this may have led to a gap in evidence regarding "who is represented and who is overlooked." This study systematically reviewed 235 empirical papers published in 2004-2024 following PRISMA 2020 to examine epistemic equity. The Equity Bias Framework was applied to operationalize epistemic equity by assessing imbalances in study design, psychometric measures, and population sampling. Results showed that the cross-sectional design, self-report, and community-dwelling adults combination accounted for the largest share (n = 99, 27.8%), whereas only 9 combinations in total (2.5%) involved clinical populations. The experimental design × self-report × university student patterns (n = 14, 3.9%) outnumber all experimental studies involving age-disadvantaged or clinical groups (n = 4, 1.1%). These patterns indicate that existing research evidence is shaped more by feasibility and accessibility than by representativeness and clinical relevance, raising concerns about epistemic equity. By introducing the Equity Bias Framework, this study provides a lens on current evidence and direction for research and policy promoting methodological diversity and sample inclusivity.