Abstract: Dynamic deflection is a critical parameter to evaluate the overall structural performance of a bridge，and it is very important to carry out a scientific and reliable test on it. However，there are few methods and equipment for dynamic testing of the bridge，especially the direct measurement of bridge dynamic deflection is still lack of scientific and convenient methods. The conventional measurement methods such as leveling， dial indicator，and total station are difficult for dynamic measurement；even though GPS can realize the dynamic measurement，the accuracy is limited，and the debugging and installation are very tedious. Based on the theory of digital image processing technology，this paper conducts a comparison experiment on photoelectric deflectometer instruments and health monitoring deflection in Wuhan Sino-French Friendship Bridge. The results show that test results are consistent with the existing health monitoring system of the bridge，which verifies the accuracy and reliability of the system，which also can meet the requirements of bridge health monitoring.

Key words: digital image processing, dynamic deflection, detection system, comparative analysis, bridge health monitoring