Aiming at the increasingly significant problem of various natural disasters affecting power grid, a natural disaster risk assessment method for power grid is given in this paper. Combining the qualitative and quantitative analysis, improved analytic hierarchy process(AHP) is used to construct the hierarchical structure model of risk assessment, the weights of factors are calculated and then the degree of risk is determined. An example application shows the effectiveness of the assessment results. It will boost the power grid's capability to withstand natural disaster.