Knowledge management is the energy to get competitive advantage for enterprises, the cornerstone to get sustainable development .But k nowledge management is one kind of actions with h igh risk s, if the enterprises want to reduce and avoid loss, they must identify all types of risk s in the process of k nowledge management effectively, supply prior - warning signals for enterprises development to guarantee their safety by using scientific ev aluating methods .The paper sets up multidimensional model of matter - element and extension set for evaluating comprehensive risk s on k nowledge management by theories of matter - element and extension set, offers extensive methods for evaluating comprehensive risk s on knowledge management .The method can consider all risk s in the process of knowledge management comprehensively, supply evidence for enterprises’ business decision .Both theories and positive analysis fruits prove that the model is practicable an d effective , it supplies a new road for evaluating k nowledge management risk s.