Abstract: For many hydropower stations in running, diversion tunnels play a role of draining accumulated water in underground space, so it is necessary to dynamically monitor the variation trend of the accumulated water. Through fully studying the key points and difficulties of monitoring the water inflow from diversion tunnels, and combining with the advantages and disadvantages of existing research results and technologies, a real time computing method based on LiDAR technology and Canny water level monitoring was proposed to calculate the accumulated water volume in diversion tunnels. Firstly, the three-dimensional spatial point cloud data of the diversion tunnel was obtained by LiDAR scanning. Then Global Mapper 17 software was used to calculate the possible water volume at different elevations, and a mathematical relationship between the water level and the water volume was obtained. Finally, the Canny algorithm was used to identify the edge of the water level in the monitoring screen of the diversion tunnel, and the pixel distance was converted into the actual height which was substituted into the water level-available water volume relationship to obtain the current water volume. Through practical application, the presented method overcomes the problem of irregular spatial structure of diversion tunnels, and can accurately establish the relationship between the water level of the diversion tunnel and the volume of available water. At the same time, the error between Canny water level monitoring and the actual result was controlled within 1%. The results have clear application values and practical guiding significance.