Urban greenways, as an important form of green infrastructure, play a vital role in improving the urban ecological environment and mitigating the urban heat island effect. However, research on the relationship between thermal comfort in greenways and human activity has been limited. Traditional thermal comfort studies have mainly focused on fixed open spaces, such as parks and plazas. Due to their linear layout and high mobility, urban greenways differ significantly in usage and thermal environment impacts. This study focuses on the Mianyang Riverside Greenway as the research area and analyzes the impact of thermal comfort on the behavior of greenway users based on microclimate data and questionnaire survey results. Using the Physiological Equivalent Temperature (PET) as an evaluation index, the study explores the correlation between activity intensity, duration, spatial preferences, and thermal comfort across different activity intensity groups. The results indicate that as PET increases, the average activity intensity in summer greenways decreases significantly. The demand for a suitable thermal environment varies depending on the intensity of the activities, with thermal sensitivity ranked as high-intensity > moderate-intensity > low-intensity activities. Higher-intensity activities require a more favorable thermal environment. Improving the thermal environment of greenways can enhance activity intensity and participation, thereby maximizing the health-promoting benefits of urban greenways. This study enriches the research content on thermal comfort in urban greenways from the perspective of activity type differences, providing practical evidence for achieving sustainable urban development.