Urban park green space has indispensable ecological value as a significant environmental resource with carbon sequestration within the city. In addition, it plays a crucial role in mitigating and responding to global climate change. To explore the critical characteristic factors affecting the carbon sequestration efficiency of the urban park green space plant community and effectively improve the carbon sequestration performance of the urban ecosystem, a total of 72 plots were selected from urban park green space in Hangzhou Economic and Technological Development Zone. SPSS software was used to analyze the relationship between plant community distinguishing factors (proportion of deciduous species, mean diameter class, mean crown magnitude, mean carbon sequestration capacity, and plot density) and carbon sequestration efficiency. The results showed that: (1) Evergreen trees and deciduous trees had the same contribution to the carbon sink efficiency of plant communities; (2) When the diameter class range is 10~20 cm, the average diameter class has a significant positive impact on the carbon sink efficiency of plant communities; (3) The average canopy width has a significant positive impact on the carbon sink efficiency of plant communities; (4) The average carbon sequestration capacity has a significant positive impact on the carbon sequestration efficiency of plant communities. The higher the proportion of high carbon sequestration tree species in plant communities, the higher the carbon sequestration efficiency; (5) Although community density is the essential condition for achieving high carbon sequestration efficiency and has a specific role in promoting carbon sequestration efficiency, high density does not necessarily have high carbon sequestration efficiency. Low-density plant communities can also have high carbon sequestration efficiency through reasonable collocation. At the end of the study, the optimization design strategy of plant communities with high carbon sinks in urban park green space was proposed to provide a reference for the subsequent research and construction of carbon sink capacity of urban park green space and improve the contribution of urban green space carbon sink to China’s carbon neutral goal in 2060.