Research on carbon source and sink accounting of urban green spaces is beneficial for guiding the rational construction of urban carbon reservoirs and scientific assessment of carbon sinks. Based on methods and accounting models for quantifying carbon sources and sinks throughout the entire lifecycle of urban green spaces, this study compares the carbon sources. It sinks over a 50-year lifecycle within the Longhu Outer Ring of Zhengzhou. The results show an imbalance between carbon sources and sinks in the initial stages of construction, with a balance achieved nearly 20 years into the lifecycle, followed by positive ecological benefits in the subsequent 30 years. During the lifecycle, the material and maintenance stages are the primary sources of carbon sources, while trees and shrubs play a crucial role as carbon sinks in the study area. This study adopts various carbon accounting methods to evaluate and predict the carbon source and sink status of urban green spaces to provide support for improving the carbon sink function of urban green spaces and achieving the goal of “carbon neutrality” in cities.