Field test and analysis were carried out for dynamic responses on a continuous bridge under open traffic. The inspection, testing and analysis for the tested bridge mainly consist of the measurement of pavement roughness and analysis on power spectral density, the testing and analysis on natural vibration characteristics, the observation of traffic loads under open traffic and the corresponding measurements of dynamic displacement and acceleration on bridge. To determine impact factors in terms of measured dynamic displacement responses, low pass Butterworth filter is used for filtering the dynamic part of measured dynamic displacement and the static extreme values are reserved. Finally, the relationship between impact factors and vehicle weight, vehicle speed is studied. It was found that impact factor tends to decrease with increasing vehicle weight, and integrally speaking, the impact factor increases gradually as the vehicle speed is increased within a wide range.