To analyze the influencing mechanism between traffic performance index (TPI) and emission factors, the speed distribution was taken as the connection variable.Firstly, a method for calculating speed distributions on different TPIs was established, and the relationship between the speed and emission factors was analyzed.Secondly, a method to calculate emission factors on road network was developed based on different TPIs, and a quantification method was analyzed to estimate the uncertainties.Thirdly, traffic emission factors on different TPIs were calculated for Beijing city.Results show that when the TPI is between 2 and 8, CO₂ emission factor increases monotonously with the increase of TPI.However, when TPI is between 0.6 and 2 or between 8 and 9.4, CO₂ emission factor increases rapidly.Finally, the average deviation rates of emission factors under different clustering results were calculated.It was concluded that by classifying the data from 4 aspects, workday, weekend, AM, and PM, the deviation rate of emission factors CO₂, CO, HC, NOx decreased from 3.48%, 6.67%, 6.08%, 6.68% to 2.65%, 5.35%, 5.20%, 5.36% respectively.