Abstract: The landforms in Kangding City are deep-cutting valley, with steep slopes and exposed bedrocks, and the falling rocks and earthquakes occurred frequently in this area. In order to carry out the study on the characte-ristics and laws of the collapse movement under the earthquake action, the researchers in this paper take rockfalls in Guoda Mountain of Kanding City as a case study and adopt the Particle Flow Code software PFC^2D to simulate the movement characteristics and failure process rockfall at different locations (boulder stone on the top of slope, catallactic rock mass on the slope, collapse accumulation in the middle of slope, block rockfall under the slope) under Ms 8.0 earthquake. The results are as following: ① The boulder stones on the top of slope with small mass are easy to be started, and the boulder stones with nearly spherical shape are easy to be capsized and rolled. The movement types after departing along the free face are falling, colliding and rolling, with the highest rate of 11.8 m/s. ② The failure process of catallactic rock mass on the slope can be divided into four stages, including extending - penetrating of fracture, starting - falling, crashing - disintegrating, and rolling - stacking. The block rock mass above the catallactic rock mass moves furthest, up to 269 m. ③ The collapse accumulation forms a debris flow from front to back, and its movement types along the slope are rolling and colliding. ④ The movement characteristics of fallrocks in the lower part of the mountain are starting, sliding, squeezing, disintegrating, re-sliding, re-squeezing and accumulating. ⑤ The collision, friction, extrusion and disintegration occurred in the collapses in different locations during movement, which quickly consumed their own kinetic energy and caused sharply dropping in distance and speed. The discrete element simulations could help refined understanding of collapse in deep-cutting valleys, which provides some scientific basis for the project management of collapse and disaster prevention in mountainous regions and cities.