In urban integrated green infrastructure, plant community landscapes can significantly mitigate the heat island effect through plants’ cooling and humidifying effects. However, existing studies mainly focus on the structural characteristics of park green spaces, with relatively insufficient research on evaluating park green landscapes about the heat island effect. Using the Baitang Ecological Botanical Garden in Suzhou as an example, this study selected 22 community sample plots. Criteria layers included species diversity, thermal comfort, and air particulates. In contrast, indicator layers comprised the number of species individuals, species richness, ecological dominance index, Shannon-Wiener diversity index, Pielou evenness index, comprehensive comfort index, and PM2.5 values. By combining the Analytic Hierarchy Process (AHP) and Principal Component Analysis (PCA), key indicators and their weights affecting landscape suitability were determined, and a landscape suitability evaluation system oriented towards the heat island effect was constructed to explore the role of plant community landscapes in mitigating the heat island effect. The evaluation results showed that the Shannon-Wiener diversity index had the highest weight contribution rate at 22.48%. In the Baitang Botanical Garden, 55% of the plant landscape units had high suitability, with tree species diversity and the ratio of trees to shrubs significantly impacting landscape suitability. Specifically, when the number of tree species exceeded five, and the tree-to-shrub ratio was close to 6∶5, the landscape units had high suitability. Based on the research results, optimization strategies from three aspects, regionality, diversity, and hierarchy, were proposed to provide references for plant landscape planning under urban heat island conditions.