Abstract: Debris flow risk study has a great significance for the engineering harnessing, as well as the determination of disaster prevention and mitigation countermeasures for Lanzhou city. In this paper, the data of typical debris flows have been investigated and analyzed, the quantitative indexs for Lanzhou debris flow disaster evaluation have been obtained through using the method of grey relational analysis, and the weights of every index have been calculated. On the basis of the evaluation model, the risk of debris flow in Lanzhou city has been evaluated. And then, the calculated risk degree of these 12 debris flows has been taken as testing indexes. It's found that the evaluation results fit well with the theoretical results, showing this method is reliable, simple and practical.