Traditionally, independent dam or tidal fences in the sea should be built before the tidal energy electric power generating. In the near future, more and more cross sea bridges will be constructed between the mainland and the islands in the east of China, or between the islands. If the tidal fences are designed to be installed on these bridges, much investment will be saved than constructing tidal fences independently. But it will increase the loads of the bridges. Considering the bridges with or without tidal fences installed, three types of cross sea bridge models are built numerically. The stresses and deformations of the bridge structures are calculated. And the effects of the fences on the bridge structure are evaluated. The results may be useful for the designing of cross sea bridges and power generating device of tidal energy.