Long-span structure is a more common structural style in the public buildings, and its seismic response often differs from other structures because of the long-span characteristic, and thereby the multi-support earthquake excitation, such as the displacement model and acceleration model, need to be adopted in the seismic analysis. However, relative researches have proved that there exists certain problem in the displacement model. According to previous works, the error causes of displacement model are described firstly and expression is also given to estimate the magnitude of error. Then, the influencing factors on the error of displacement model are also given and analyzed. In the end, numerical studies are carried out to verify the viewpoint proposed in this paper.