Abstract: Photovoltaic (PV) power generation is significantly influenced by climatic factors. To rationally assess the rooftop PV potential in China, this study selected five cities with abundant rooftop resources in distinct climatic zones—Haikou, Kunming, Xining, Jinan, and Hohhot—and evaluated the PV generation potential and environmental benefits of their downtowns using GIS technology. The results indicate that the utilizable rooftop area for PV installations accounts for 34%–40% of the total rooftop area in the study regions. The annual potential PV generation in the central urban areas of Haikou, Kunming, Xining, Jinan, and Hohhot is estimated at 1 369.25 GWh, 1 278.10 GWh, 923.36 GWh, 3 213.88 GWh, and 3 150.76 GWh, respectively. These outputs could meet 122.08%, 30.78%, 61.47%, 74.74%, and 174.57% of the residential electricity demand in the corresponding cities. Under equivalent substitution of coal-fired power generation, the estimated carbon emission reductions reach 0.862×10⁶ tons, 0.805×10⁶ tons, 0.707×10⁶ tons, 2.496×10⁶ tons, and 2.447×10⁶ tons, respectively. In conclusion, rooftop PV systems can effectively reduce reliance on conventional energy sources, mitigate climate change impacts, enhance renewable energy penetration rates, and lower carbon emission intensity in China.