Tree-lined spaces as informal communication areas and important pathways for pedestrians are the second largest zones on university campuses, and they have a large impact on the microclimate. At present, the effects of the spatial form for tree-lined boulevards on microclimates have not been investigated. Thus, this study applied experimental and simulation methods to investigate the effects of tree-lined boulevards on microclimates in hot summer and cold winter regions. The main meteorological parameters including air temperature, relative humidity, wind speed, and solar radiation of the boulevard were obtained by experiments. Furthermore, the experimental data as a boundary condition were input into ENVI-met software to investigate the effects of the aspect ratio and canopy diameter of double-row open-canopy boulevards on microclimate regulation. The results showed that when the aspect ratio was reduced from 1.5 to 0.9, the temperature and UTCI increased by 0.047 °C and 0.21 °C, while relative humidity decreased by 0.227%. Decreasing the aspect ratio can effectively improve the microenvironment. As the canopy diameter increased from 7 m to 11 m, the temperature and UTCI of the boulevard space decreased by 0.064 °C and 0.45 °C, while relative humidity increased by 0.245%. An increase in canopy diameter is unfavorable to the improvement of microclimates. This study aims to provide a scientific basis for the design and improvement of tree-lined boulevards on university campuses.