A research has been carried out on the influence of urban canopy morphology on the migration and diffusion features of radionuclides by using the method of numerical simulation, with an introduction of such four indicators as the velocity ratio (VR), age of air (τp), retention time (τc)and atmospheric dispersion factor (ADF), for an evaluation of the flow and diffusion of radionuclides at pedestrian altitude in urban canopy. The numerical results indicate that changes in urban canopy morphology have a relatively small impact on the air age on the main streets, but have a greater impact on the air age within the street canyons. There is a stronger washing air on the windward side of higher buildings with a better ventilation, while there is airflow stagnation on the leeward side accompanied by a poor ventilation. The migration and diffusion of radioactive isotopes are mainly under the influence of the morphology of urban canopies. Street canyons with a good ventilation may not have a shorter residence time, while poorly ventilated street canyons may not necessarily have a longer residence time. Due to the influence of mechanical turbulence of buildings, the atmospheric dispersion factor is obviously reduced. When the urban canopy shows a protuberant and ascending tendency, the radionuclides are characterized with the best diffusion effect within an area of 450~900 m downstream.