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Impact factor is amplification factor of vertical dynamic effect produced by vehicles. It is a main parameter of bridge design and an important index of dynamic load effect evaluation. In order to study the influence of structure and excitation factors on the impact factor of highway bridges, and then obtain the real impact factor of the continuous beam arch composite bridge, taking a three-span arch bridge made with continuous composite concrete filled steel tube beams as an example, considering the vehicle-bridge coupling vibration effect, the spatial beam element model of the bridge and the half vehicle model with the three-axis are established by using ANSYS. The impact factor of different parts of the main beam and different responses affected by the deck surface roughness, the vehicle speed and the number of vehicles are analyzed. The binary regression formula of impact factor is obtained by taking the vehicle speed and the roughness of bridge deck as independent variables. Finally, the formula is verified by the measured data of two bridges with similar fundamental frequencies. The results show that the impact factor calculated by the current code is generally small for the bridge structure with complex structure and relatively low frequency, such as arch bridge made with continuous composite concrete filled steel tube beams. The impact factor is most affected by the roughness of bridge deck. When the roughness of bridge deck reaches grade B or above, the impact factor exceeds the specification value, and the maximum impact factor can reach 5.42 times of the specification value. For the main beam, the impact factors of different external excitation, different responses and different parts are not the same, and some impact factors exceed the specification value. The regression formula of impact factor given can be used to estimate the impact factor of main beams of similar structures.