In the context of the Double Carbon Development in China, the combined direct sink and indirect carbon reduction benefits of urban green space are outstanding for the core areas of large cities with high-density concentrations. This paper takes the Xinjiekou district of Beijing as the research object and conducts a survey on the refined green space of street space. We take 18 streets as samples, accurately locate and record 3242 street trees, apply the i-Tree Streets model to assess and calculate the comprehensive benefits of low-carbon street space, and come up with recommendations for the refined transformation and enhancement of street space. The results of the study show that the street trees surveyed in the Xinjiekou area belong to 18 families, 21 genera, and 26 species. The largest proportion of Sophora japonica was found in terms of quantity, accounting for 74% of the total number of trees surveyed. The total ecological benefit value of the street was 13,577,110,000 yuan, with an overall average weighted benefit level of 4,188.01 yuan per plant for a single plant. Among them, carbon storage benefits accounted for 67.40%, while other benefits accounted for aesthetic benefits, CO2 absorption benefits, energy saving benefits, rainwater retention benefits, and air quality improvement benefits in that order. Through the analysis of the evaluation results and problems of the comprehensive benefit value, the strategies and suggestions for improving the refined low-carbon transformation of urban street space are proposed from three aspects: the comprehensive analysis of tree species benefit, the improvement of indirect carbon reduction benefit, and the maintenance and management of green space environment throughout the life cycle.