As a typical plant allergen, plant flotsam causes many negative effects on urban production, residents" daily life and public health. Existing research mostly focuses on species identity of allergenic tree, spatiotemporal patterns of allergen concentrations, and assessment of allergenic risks. However, it rarely involves the impacts of streetscape characteristics and local environmental factors, as well as pedestrians" perceptions. In addition, studying flotsam as a representative of plant allergens is more economical, practical, and universal than conducting research on pollen. The dominant factors of near-surface concentration of plant flotsam were quantified including the street structure，streetscape spatial patterns, street tree characteristics, and microclimate of 15 streets involving main streets, secondary streets and branch streets in Hangzhou, across different urban functional zones such as residential areas, commercial areas, and tourist areas. Raw data was collected by field work, landscape analysis and questionnaire survey. The correlation between public perception of plant-caused allergy and flotsam concentration was also explored. Index of urban green zone allergenicity was applied to assess the street allergic risk. A sankey diagram based on questionnaire data was used to show the variation in allergic symptoms of pedestrians with given period and weather conditions. Finally, the relationships between public perception of allergy, observed flotsam concentration and the variables of street characteristics were analyzed by the generalized additive model and variation partition. The results showed that: (1) there was a positive correlation between the public perception of allergy, index of urban green zone allergenicity and observed flotsam concentration on the streets; (2) concentration of flotsam was positively associated with street tree’s diameter at breast height and their interval distance, and negatively associated with motorway width, surrounding woodland area, and building area, and a U-shaped relationship observed for humidity, with a stronger effect of landscape characteristics than plant characteristics and microclimate; (3) allergy incidence rate was positively correlated with air temperature, street tree’s interval distance, and surrounding woodland area, and showed unimodal relationships with humidity, flotsam concentration, and water body area, with a strongest effect of plant characteristics; (4) allergy perception was negatively correlated with flotsam concentration, street tree’s interval distance, and surrounding woodland area, and exhibited U-shaped relationships with temperature and water body area. Based on these findings, four street-level spatial design and management strategies, such as optimization of street tree species composition and streetscape spatial structure, the artificial regulation of microclimate, and behavioral intervention for allergen-susceptible groups were proposed to mitigate the allergy risks posed by plant flotsam in urban streets.