Trung Son hydropower project was built on the Ma river, which plays an important role in providing energy for Thanh Hoa province. During the operation this power, there was a landslide of the excavated roof downstream of the right shoulder of the dam, threatening the overall stability of the damp. This paper presents the results of analysis of the instability of the excavated roof downstream of the right shoulder of the Trung Son hydropower project. The investigation results show that, in the area of excavated roof downstream, a large–scale landslide appeared, over time the sliding developed, threatening to the dam. According to the assessment, the main cause of instability is because the roof is made of schist, the schist surface is in the same direction as that of the excavated roof, the dam foundation has faults, under the effect of rainfall, the durability of soil and rock is reduced, causing instability of the slope. In order to treatment, the proposed soft anchor drilling solution, the anchor length is designed into zone IIA, exceeding the specified slip arc range. Calculation results on Geoslope software in cases of processing at different sections give long–term stable for excavated roof downstream.