According to the uncertainty, the influence of subjective weighting and the fuzziness of subordinate interval of criterion layer and index layer in dam safety diagnosis, a dam safety synthesis diagnosis model based on Fuzzy cloud theory is put forward. Taking an operating dam as the object of diagnosis， on the basis of the dam safety assessment index system, the weight matrix was determined by using the weighting theory of the improved analytic hierarchy process, the membership matrix is solved by means of the cloud model to quantify the randomness and fuzziness in dam behavior diagnosis. Then, the results of fuzzy mathematical operation of the above matrix are evaluated according to the principle of maximum membership degree. Finally, the model is applied to an actual dam project, the safety diagnosis results of domestic scholars and analysis results are basically the same, and the results are consistent with the actual service conditions, which verifies the feasibility and effectiveness of this model.