The main dam landslide of certain dangerous reservoir occurred in the course of reinforcement process. Through the analysis of engineering geological conditions, construction method, construction process and construction management, the causes of landslide are acquired. The causes of the landslide are concluded as follows： 1. The construction quality of the dam body was bad. 2. The construction method of the cut-off wall was not appropriate. 3. Adopting emergent measures were not in time after the dangerous status appearance. 4. Pore water pressure inside the dam slope did not vanish when the water level of the reservoir declined suddenly. In combination with the actual situation in the project, contingent measures and reinforcement program were put forward. By following reinforcement , the reservoir is running in good condition. The experience and lessons summed up can be used as a reference for other similar projects.