Accompanied by the expansion of cities worldwide and the massive use of fossil fuels, ever-increasing carbon dioxide (CO₂) emissions in the urban areas contributes to global climate change. Studies from urban forest have reported that quantification of forest biomass and photosynthetic rate are of great significance to forest carbon cycle studies and climate-change mitigation. However, most former studies have focused on calculating CO₂ sequestration of urban forests at regional and urban scale, and few reports are available in the spatial differences of carbon fixation and the efficacy on offsetting energy CO₂ emissions in urban areas at finer scale. Therefore, based on field survey data and urban forests data derived from high-resolution Gaofen No.2 images, this paper modeled the spatial changes of CO₂ fixation by using assimilation method and biomass method in Shanghai, the most prosperous metropolitan area in China. We then mapped CO₂ emission in pixel size based on yearbook data, population distribution and NPP-VⅡRS nighttime light data. The results showed that the urban forest in Shanghai could annually absorb 1.36 million tons of CO₂, and the average CO₂ fixation capacity reached 17.02 tons per hectare. The CO₂ fixation capacity varied among different urban forest types. The broadleaved forest ranked first (18.22 t/hm²) while the conifer forest manifested the lowest CO₂ fixation capacity (7.11 t/hm²). The spatial distribution of CO₂ fixation presented a significantly uneven overall spatial distribution: low in the middle and high on all sides, and the forest in Chongming District and Pudong New District jointly contributed 52.8% of the total sequestrated CO₂. In addition, the results indicated that the amount of CO₂ emission from annual energy consumption reached 0.27 billion tons in 2017 and the amount of CO₂ emission from center area occupied 11.49% of the total CO₂ emission, being spatially high at center and low at fringe. Collectively, the annual CO₂ sequestration of urban forest could only offset 0.5% of the annual energy CO₂ emissions in Shanghai. Furthermore, the CO₂ sequestration spatially mismatched the CO₂ emission. Only 0.08% of energy CO₂ emission in the center area could be offset by its urban forest. Therefore, these results can be used to help assess the actual role of urban forests in reducing atmospheric CO₂ in Shanghai, and we suggested Shanghai should promote the carbon dioxide fixation capacity of urban forests in the center area, and strengthen the observation and utilization of carbon dioxide sequestration in the marine ecosystem.