Urban green spaces have received more and more attention for rare and endangered species conservation. However, the conservation potential in urban green spaces has been less studied. Based on the habitat survey of Changium smyrnioides in cities Hangzhou and Ningbo and the eco-physiological experiment, we determined the temporal niche of this species. Then we investigated the urban green spaces to search the habitats that are similar to that in the field and analyzed the potential and the way for reintroducing C. smyrnioides into urban green spaces. Results show that C. smyrnioides mainly occurs under the deciduous trees in eastern subtropical forest. When the deciduous trees are with no leaves from November to next April, the understory light and temperature conditions are suitable for the growth of C. smyrnioides. In Hangzhou and Ningbo, C. smyrnioides begins to germinate during December and January, and enter a period of high photosynthesis and rapid growth with optimal temperature and light in March; it blooms and the photosynthesis becomes weak in May; it enters dormancy after seed maturation in June. In urban green spaces of Hangzhou and Ningbo, the deciduous tree species account for 62% and 60% of the total number of tree species, respectively. The spaces under these deciduous trees have the potential for C. smyrnioides cultivation. Moreover, the understory habitats of the important deciduous species that similar to the wild species are most suitable for C. smyrnioides reintroduction. At present, there are 18.97 and 18.42 km² of potential habitats for C. smyrnioides in urban green spaces of Hangzhou and Ningbo respectively, and if the potential habitats are taken full use of, 1.01×10⁸ and 0.98×10⁸ individuals of C. smyrnioides can be cultivated, respectively.