Under the framework of Social-Ecological System(SES), the assessment of ecosystem adaptive capability is based on three factors: exposure index, coping capacity and recovering capacity. This paper presents a ecosystem adaptive capability model, ACI=CC·RC/EI (ACI is Adaptive Capacity Index; EI, CC, RC is Exposure Index, Coping Capacity, and Recovering Capacity respectively), to assess quantitatively ecosystem vulnerability. Then the ecosystem adaptive capability to soil erosion in Lanzhou is assessed based on the model. The results show that 74% of Lanzhou land has a low or medium adaptive capability to soil erosion, with adaptive capability index ranging from 0.06 to 0.61. As a whole, the adaptive capability to soil erosion in Lanzhou is presented as high exposure index, low coping capacity and low recovering capacity.