Abstract: The vehicle synthesis emission factors are the foundation of calculating total emission quantity and emission sharing rate of urban vehicle pollutants, the important basis for reducing the urban vehicle emissions, as well as the source to control the urban road transportation pollution. Through using the research results of the vehicle operating modes on urban roads of Ningbo City, the emission intensities of main pollutants such as HC, CO and NO_x of testing cars under the urban road operating mode in Ningbo were obtained on the acceleration simulation operating mode (ASM) emission test system. The testing car pollution discharge values were calculated based on testing car design parameters of the transmission gearbox and the main gear box and on the transmission gearbox positiondistribution when the testing car moving on the urban roads of Ningbo. According to investigation results of vehicle age distribution and vehicle type distribution of the urban road transportation flow, the revision to the testing car pollution emission values was carried on. By the method, the urban road vehicle synthesis emission factors in Ningbo were obtained, being 5.89, 21.22 and 18.91 g/(km·veh). respectively for HC, CO and NO_x.