| "5.12" Wenchuan Earthquake created not only a large number of collapse and landslide hazard in the earthquake affected areas, but also 104 dammed lakes formed by landslides blocking the river in the core area. The destruction of slope deformation in the reservoir area had increased in various degrees. It is destructed strongly particularly from Tangjiashan to Xuanping Village. The destructed situation and the stability of Malingyan landslide in Tangjiashan dammed lake affected the feasibility and availability of the comprehensive improvement of Tangjiashan dammed lake in a certain extent.Malingyan Landslide is located in the left bank of Tongkou River, which is 4 km away from the upstream of Tangjiashan dammed lake. Because of its special location, it receives much concern in the process of dealing with an emergency of the dammed lake. In this paper the author has collected a large number of basic Malingyan landslide data, and has already conducted a field survey of landslide as well as obtaining the corresponding data, including survey of landslide's basic topographic feature, survey of landslide's structure feature, hydrological exploration of the landslide and its surrounding, survey of landslide's size and characteristics. Combining a series of the landslide stability analysis and evaluation methods in and abroad, Malingyan landslide stability is researched systematically. According to the topographic features on the Malingyan landslide profile, the landslide can be divided into three periods. Limit equilibrium methods are used to calculate the stability on the three periods'landslides in the General conditions of the natural state, natural and earthquake, storm conditions, storm and earthquakes. And geo-slope numerical simulation method is used to analyze the numerical simulation of the three periods'landslides on the natural state and seismic conditions. In the paper, through the researches of Malingyan landslide Causes and stability factors as well as the comprehensive analysis of the stability after "Wenchuan" earthquake, the methods of stability calculation and numerical simulation are provided.Based on the field geological investigations of basic landslide geological conditions, deformation and failure characteristics before and after Wenchuan 8.0 earthquake, stability of Malingyan landslide, especially after the formation of dammed lake, is analyzed. The results show that the landslide is generally stable after earthquake, but part of the former section of the upriver is possibly instable because of earthquake and water storage of the dammed lake and so on. Malingyan landslide will not put against Tangjiashan dammed lake and the comprehensive treatment at a disadvantage in the future can be saved. And then, the potential sliding on the upside may be harmful tosome residents on the slope body. Based on analytical stability results of Malingyan and the other landslides in earthquake areas, the overall reactivation of the large-scale ancient landslide caused by earthquake is mainly controlled by its terrain slope and micro-geomorphic characteristics. The whole or part ancient landslides triggered by earthquake will be instable if it satisfies the sufficient condition, in which the terrain slope is above 40°as well as slow part changes into steep one. Not all ancient landslides are possibly reactivated by the earthquake, and this will tremendously change the common misunderstanding in the past that the earthquake will trigger the whole ancient landslide reactivated inevitably.
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