In order to detect the abnormal crowd behavior in video surveillance in real time and more accurate, this study proposes a method of dynamic interframe space updating based on the Pyramid LK optical flow. The algorithm dynamically updates the interframe interval by extracting the crowd motion information, and then detects the crowd motion information at the interframe interval. In this way, the algorithm does not only preserve the advantages of the traditional algorithm in detecting crowd motion information, but also improves the efficiency. Finally, the algorithm identifies the abnormal crowd behavior by acquiring the intersection density and energy information of the crowd motion vector. By testing multiple videos, the test results show that the algorithm can identify the abnormal crowd behavior in the video with high accuracy, and also effectively improves the running speed.