This paper presents the field geological investigation, engineering geological exploration and other related studies on the collapsed and slided debris mass induced by the Xiaonanhai Earthquake in Chongqing city. Basic characteristics and formation mechanism of the debris mass are elaborated. It is held that the forming process of the mass consists of four steps: namely weathering and denudation, shocking and projecting, collapse-slide accumulating and dammed lake formation. Collapses rather than landslides appeared in process of rock mass fracturing and rock blocks in accumulation body are quite and even more fragmental. Research shows that the mass is a result of comprehensive actions including seismotectonic moving and stress field, rock mass structure, topography and geomorphology, and seismic wave. First of all, the left lateral tensile-slip fracture in Xiaonanhai fault block was induced by X-shaped seismotectonic combination. It consists of the NE trending right lateral Qianjiang reverse-slip fracture and the NNW trending left lateral Yangtoushan reverse-slip fault. The NWW trending axially tectonic compression resulted in the M61/4 earthquake. Secondly, the rock mass near both Dakuayan and Xiaokuayan with three groups of tectonic joints forming X-shape collapsed respectively towards their free faces, at about 145~155 direction. Thirdly, the limestone rubbles discovered in the collapse-slide mass prove that mountain body before moving contains limestone belong Qixia and Maokou formations during the Permian period. This discovery provides the evidence for restoring mountain body before its collapse-slide. Finally, the diameter of rock blocks in accumulation body decreases from northwest to southeast along 150 direction. I It proves indirectly that the direction of rock mass ejecting is along 150o direction. What's more, the collapse-slide masses from mountain body near Dakuayan and Xiaokuayan form their accumulation bodies, respectively. Therefore their overlapping part is small.