Applying the principle of tunnel test to run a typical across-river tunnel test in Guangzhou city, 48 h-online-monitor data include pollutant concentration, traffic activity and meteorological data were gained. The average motor vehicle emission factors of NOx, CO, SO2, PM10 and HC were calculated using mass balance which are 1.379, 15.404, 0.142, 0.637, 1.857 g/km. vehicle respectively. Based on that, combined emission factors of 8 types of city vehicles were calculated using linear regression. The result basically showed the character and level of motor vehicle emission in Chinese city.