Under the dual pressures of global warming and accelerated urbanization, urban green spaces (UGS) serve as crucial yet paradoxical elements, alleviating urban heat island (UHI) effects while emitting biogenic volatile organic compounds (BVOCs) that exacerbate air pollution; however, their spatial trade-offs remain underexplored. This study bridges this gap by developing an Urban Heat Mitigation Index (HMI) and a BVOC flux accounting framework integrating remote sensing and field observations. The results showed that (1) the cooling effect exhibits significant spatial heterogeneity, with continuous green networks around West Lake and along the Qiantang River forming efficient cooling corridors (HMI > 0.75), while fragmented green spaces in northeastern areas show weaker cooling effects (HMI < 0.35); (2) BVOC emission intensity displays a “high suburbs-low centers” pattern, with suburban areas emitting 1.9–2.3 times more BVOCs than urban centers, while BVOC-induced PM2.5 (0.02–0.05 μg m−3) and O3 (12–33 μg m−3) concentrations in city centers still pose significant health risks; (3) spatial analysis reveals a weak positive correlation between HMI and BVOC emissions (Moran’s I = 0.096, p < 0.05), with four distinct coupling patterns identified: “high cooling-low emissions” (17.5% of area), “low cooling-high emissions” (1.1%), “high cooling-high emissions” (18.7%), and “low cooling-low emissions” (3.9%). This study provides quantitative evidence for optimizing UGS layouts to balance ecological benefits and environmental risks, emphasizing the importance of vegetation screening and spatial allocation in sustainable urban planning.