A simply supported girder bridge built in Fujian Province was taken as a research background. The bridge was transformed into a semi-rigid integral bridge in the practical engineering. On the basis, the original bridge would be transformed into an integral bridge, semi-integral bridge, and extended-deck bridge. The finite element models of five bridges were built based on the MIDAS/Civil software. The differences of their mechanical performances under the seismic load were analyzed.The results show that the simply supported girder bridge is easy to cause the girder to fall at the abutment, while the jointless bridges can effectively prevent the occurrence of the phenomenon. The integral bridge shows a better seismic performance and is more suitable for building in the high earthquake areas. The effective lengths of piles of the jointless bridges and simply supported girder bridge are at the buried depths of 0-10D(D is the pile diameter). The pile of integral bridge has a good mechanical performance under the large seismic load, which can be better for protecting the pile foundation from damage. The mechanical behaviors of the pile at the abutment bottom of the extended-deck bridge and simply supported girder bridge are similar. Therefore, the design of the extended-deck bridge can refer to the design code of the traditional seam bridge. In addition, it is also found that the bending moment at the pier bottom is the largest in both the jointless bridges and simply supported girder bridge, and the plastic hinges can be easily formed at the position. Under the action of longitudinal seismic load, the most unfavorable positions of mechanical behaviors of girders of the simply supported girder bridge and extended-deck bridge are respectively appeared at the mid-span and pier top, while those of the integral bridge, semi-rigid integral bridge and semi-integral bridge are appeared at the abutment top. More attention should be paid to these positions in the design. The research results can provide a reference for the design of jointless bridges and formulation of related codes.