Abstract: The Sanjiang active tectonic zone in Southwest China is a large landslide-prone area. A giant landslide, named Chongjianghe landslide, is discovered near Luosiwan Power Station on the Chongjiang River, a branch of Jinsha River, in the northwestern area of Yunnan. The giant landslide is located at the step-fault zone of Zhongdian fault belt, belonging to a giant landslide caused by ancient earthquakes, with a volume of up to 5.8×10⁸ m³. The evolution process of the giant ancient landslide experiences a total stabilizing and local reactivations, and is characterized by grading and zoning. i.e., the giant landslide includes a number of secondary landslides. Generally, the lower its level sequence is, the poorer its stability is. On the basis of field investigation, ring shear tests are completed on the sliding soils from the front edge and the back edge of the secondary reactive landslide respectively, and the shear strengths at different parts of sliding soil under large shear displacement are obtained. The test results show that the breakage and softness of the rock mass in the active tectonic zone, as well as the stress concentration in the slope foot, are important factors for local reactivation of the Chongjianghe giant landslide. Meanwhile, the liquefaction of sliding soil caused by the excess pore water pressure in saturated or nearly saturated soils contributes to the occurrence of large deformation of slope. The test results explain that even a small amount of excavation of the highway slope foot may cause slow, sustained large-deformation phenomenon. At present, such measures as drainage and pile driving should be taken to reinforce the secondary landslide.