In order to explore the city road traffic impedance is more in line with the actual traffic flow calculation model, through the city roads will be relatively crowded in (converted into the corresponding equivalent traffic flow of motor vehicles) generated in the liquid impedance analogy in fluid mechanics in the condition of fixed pipeline facilities, flow resistance flow according to certain rules of the and, as a basis for the improvement of the widely used BPR model. The model involves the following main factors: service level, vehicle density and vehicle composition. On this basis, a complete formula is constructed and the coefficient is calibrated through actual data. Finally, the accuracy and reliability of the formula are analyzed by taking the traffic data measured at the South Second Ring Road of Xi'an as an example. The results show that considering the influence of multiple factors on the calculation of road impedance for urban roads in China.