Urban form, especially urban functional form, is an important consideration for urban planning, construction, and management. Recent progress in characterizing urban functional form makes it possible to quantify the relationship between urban functional form and urban carbon emissions. We used urban functional form data from 178 cities of China to study the relationship between urban CO2 emissions and five categories of urban form, compactness, extension, fragmentation, irregularity, and concentration. The results show that all five categories significantly affect the total CO2 emissions (TCE), and four categories (excluding fragmentation) significantly affect per-capita CO2 emissions (PCE). Compactness produces a significant negative effect on both TCE and PCE, for every 1% increase in the functional compactness index (FCI), TCE and PCE decrease by 0.79% and 0.34%, respectively. Carbon-emission-reduction policies should be combined with the topographical environment, spatial structure, and industrial structure of cities. FCI reduces carbon emissions most effectively in plain and single-center cities. The planning and control of forms are more important in hilly and mountainous cities, multi-center cities, and highly industrial cities. This study concludes that the adjustment of urban functional form has important theoretical and practical significance for low-carbon city development.