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Many cities in warm regions are predicted to become hotter and more arid due to global warming and the urban heat island (UHI) effect. This research investigated mitigation of UHI effect using small-scale plant communities in a semi-arid urban green space. We measured microclimate characteristics, including the air temperature, solar radiation, wind speed and relative humidity, at six sites in Lanzhou, China. The results indicated that it was possible to relate the humidifying and cooling effects to the area and density of the plant community. Canopy density and canopy area strongly affected the humidifying and cooling effects between 9:00 and 15:00. The humidifying and cooling effectiveness of the different plant community types may be ranked, high to low, from multilayer community -> tree-grass community -> bamboo groves -> ground cover. To measure the impact of the plant structure of the urban environment on increased comfort, we proposed a calculated model for bioclimatic comfort in the different sites. Multivariate linear regression indicated that canopy area, canopy density, tree height, and solar radiation affected humidity increase and temperature reduction. This study has significant implications for urban management, science improvement, and societal benefit.