The stable operation of power system is impacted by cascading trip-off of wind farms，it is very necessary to find out the key factors that cause the cascading trip-off of wind farms，and to assess the immunity capability of such events and put forward the corresponding improvement measures.Taking doubly fed induction generator（DFIG）as an example and focusing on dynamic voltage evolving process，a concept of voltage tolerance curve for DFIG wind turbine is proposed，and the mechanism analysis of cascading trip-off is performed.On that basis，an interval analytic hierarchy process is used to establish a hierarchical structure model of multiple factors for the immunity capability assessment.Moreover，an approach is proposed to find the key indicators depending on the selected target using the marginal indicators sensitivity analysis.Finally，case studies verify the effectiveness of proposed method and the corresponding improvement measures are proposed.