Population urbanization significantly influences urban carbon dioxide (CO2) emissions. As urban areas expand and populations grow, the demand for energy and resources rises, contributing to higher CO2 emissions. Understanding the objective laws and mechanisms linking these factors is crucial for promoting low-carbon and environmentally sustainable urban practices. This study utilizes the Spatial Autoregressive (SAR) model to explore the impact of population urbanization on CO2 emissions and innovatively investigates the roles of policies and demographic factors in this process. By examining 30 representative provincial administrative regions in China, the results indicate: (1) From 2000 to 2020, the global Moran's I index for CO2 emissions generally shows a declining trend, with high-emission areas clustering increasingly in the northern regions; (2) CO2 emissions exhibit an accelerating upward trend with the progression of population urbanization, with significant regional disparities across eastern, central, and western China; (3) Energy transition policies and the establishment of a carbon emissions trading system are effective in reducing CO2 emissions; (4) During the process of population urbanization, it is essential to consider urban environmental carrying capacity and employment guidance to ensure a fair energy transition.