In order to solve the puzzle of large video data from video monitoring system， the behavior identifying technique is adopted in the system. The input compressed video stream is decoded to image data， which is to be identified then. The identifying items include:①Is there any moving object existing in specific area?②Does any moving object run across the alert line?③Whether worker enters into substation with red helmet or not? If one or several identifying behaviors happen on image， alert information will be shown on the monitoring interface， and relevant information will be processed by many ways. Practice shows that the system has satisfied design expectation.