The invention discloses the dust fall monitoring system of a construction site. The system comprises a monitoring terminal, a spraying machine and a background monitoring center. The monitoring terminal collects the image information and the environment information of the construction site, and transmits the image information to the background monitoring center through a first wireless communicator. An analysis controller collects and analyzes the image information and the environment information, and a dust fall signal is generated if dust is detected. The spraying machine receives the dust fall signal through a second wireless communicator and carries out dust fall operation. The background monitoring center is interacted with the monitoring terminal through a third wireless communicator, and the received image information is displayed through a display terminal. A main controller receives the dust fall signal and emits an alarm signal. The main controller issues the dust fall signalto the spraying machine through the third wireless communicator. The monitoring terminal, the spraying machine and the background monitoring center are interacted through the wireless communicators.Wiring is simple, installation and maintenance cost is low, and the system is suitable for a building construction condition. The reliability of the dust fall monitoring system is high.