With the acceleration of China's urbanization, the transport sector appears one of the most important sources of SO2 emissions in Chinese cities. Compared to the fast-growing consumption of automobile, the growth rate of urban transportation investment is relative low. This study is designed to evaluate the impact of the urban transportation investment on SO2 emissions, using the data of capital cities of 30 provinces in China from 2002 to 2012. Our empirical results show that the urban transportation investment increases SO2 emissions in the short run since it might cause road blockages which would enhance the emissions of the low-speed traffic. But in the long run the urban transportation investment has a positive impact on reducing SO2 emissions as it could widen the roads and make the traffic system more accessible. Moreover, we also find that there exists a SO2 Kuznets curve in China and environmental regulation can effectively curb SO2 emissions. The robustness tests are conducted to support the empirical results. Policy suggestions are further recommended to help the urban traffic planning to reduce SO2 emissions.