In the context of rapid urbanization and increasingly diverse urban forms, urban forms significantly influence outdoor spaces’ microclimates and thermal comfort, directly affecting the frequency of people’s use of outdoor spaces. This study takes Gyeonggi-do, South Korea, as a case study. Combining on-site measurement with numerical simulations reveals the impact patterns and optimization strategies of microclimate and thermal comfort at the block-level, which are influenced by different urban morphological characteristics such as building height, building density, and green coverage. The research found that the seasonal impact of urban form on microclimate has significant temporal differences: the differences in microclimate during summer are most pronounced during the peak daily temperature, while in winter, they are particularly prominent in the morning. The dominant thermal comfort factors show seasonal changes, with sky view factor (SVF) and green space rate significantly affecting summer comfort. At the same time, in winter, they are mainly determined by the SVF and building density. In addition, the SVF has the opposite impact on thermal comfort in different seasons, indicating that building density and open spaces should be planned reasonably to improve thermal comfort throughout the year. This study provides scientific evidence for urban planning and design, helping to improve urban thermal environments and enhance residents’ quality of life.