The accuracy and applicability of horizontal seismic force of the railway girder bridge’s bearings under design earthquakes, which was calculated with the static method, were studied in this paper. The round-ended gravity piers of a railway simply-supported girder bridge with a pier height of 5-20 m and a span of 32 m was taken to established a single pier seismic model. The response spectrum method was used to analyze the horizontal seismic force of the bearings under different site conditions, and the reasonable stiffness value of pier section under design earthquake was discussed. The analysis results showed that the piers are cracked under design earthquake, so the stiffness of cracking section should be selected as the section stiffness when calculating with the response spectrum method. The design value will be too large or too small if using the static method to calculate the horizontal seismic force of bearing. With the decreasing height of pier, the site conditions become worse and the dynamic effect become more obvious. It is suggested that the horizontal seismic force of the bearing should be calculated by the dynamic method.