The invention provides a method and device for judging the state of a negative pressure system. The method includes the steps of setting up membership functions of all evaluation indexes of a unit to be judged, generating an evaluation rating set, obtaining running state values of all the evaluation indexes of the unit to be judged, setting up fuzzy similar matrixes of integrative factors of the unit to be judged according to a set weight set, the evaluation rating set and the running state values, generating fuzzy equivalent matrixes of the fuzzy similar matrixes of the integrative factors through square calculation, classifying the fuzzy equivalent matrixes according to set cutting values, and generating a judgment result for the state of the negative pressure system of the unit to be judged. According to the method and the device, the health condition of the negative pressure system of the unit can be accurately analyzed and judged, the method and the device are suitable for both water-cooled units and air-cooled units, accurate classification can be conducted on complex equipment, no omitting occurs, reliable analysis is provided for related personnel, and therefore work efficiency of a whole power plant is improved.