The "5·12" Wenchuan earthquake induced a large number of landslides，which had directly threatened the safety of human lives and properties． The old Sanjiadi landslide in Southern Gansu Province experienced largescale collapse in foreside and a long，and through cracks formed owing to the front traction during Wenchuan earthquake，which could seriously jeopardize the safety of 1 732 people of the village on it． It is therefore very important to understand adequately and maintain effectively the stability of the landslide． Based on field investigation，indoor test and related studies on Sanjiadi landslide，the basic characteristics and formation mechanism were elaborated． Assuming different working conditions，the stability and stress-strain features were calculated and analyzed by adopting transfer coefficient analysis and numerical simulation． The results show that the integral stability coefficients of the old landslides ranged 1． 30 ～1． 36，implying a steady state in the normal condition． Under rainstorm and seismic conditions，the stability coefficients of landslide were 1． 02 ～1． 09，indicating an unstable state． Finally，relevant engineering strengthening measures were provided based on the formation mechanism and quantitative evaluation results of the landslides．