Bridge operation safety is critical to national security and people's livelihood. The structural health monitoring of bridges has emerged as an increasingly active research area. Wireless sensor networks (WSNs) technology is known to be easy to deploy and inexpensive to maintain. It is thus suitable for structural health monitoring of bridges. This paper gives a survey on structural health monitoring systems based on WSNs technology. Some basic theories and typical methods in subsystems are presented, and critical technologies are analyzed. At the end, varies issues in existing systems and directions on future work are analyzed and summarized.