in this paper, we develop a bridge diagnosis system for daily bridge maintenance. The system includes the technology of wireless sensor network, signal processing, and structure analysis of a bridge. We collect the vibration data caused by running vehicles through wireless sensor network and then use transfer function to analyze the data. We cut the bridge's beam by purpose to see the difference of the Transfer Function, according to that we back step the location of the crack place. At last, we give out the result of our spot experiments and our conclusion.