In order to provide the theory basis for a low carbon transportation system set of small and medium-sized city, traffic simulation network model of the center of Qingyuan city was built with TSIS/CORSIM microscopic simulation software of the America Federal Highway Administration and combined with the central area traffic investigation of Qingyuan city ,Guangdong Province, and simulation study was made for five scenarios based on the traffic volume. Results showed that the network traffic emissions increase with the rise of traffic flow; emissions of HC, CO, NOx and carbon dioxide per kilometer increase by 8%, 12%, 12% and 29%, respectively when traffic flow increase to 1.5 times; Fatherly, when the increase traffic flow(1.5 times) led to traffic congestion, extra fuel consumption(225 L) and extra CO2 emissions(518 kg) are brought out. The extra CO2 emission is 16.3% of the total discharge.