The invention relates to the field of fault diagnosis based on models and electrical system faults, in particular to an electrical system modeling method and a fault diagnosis system based on a qualitative model. The electrical system modeling method comprises the steps of determining variables used for describing the working state of each component, defining the types and the variable instances of all the variables, defining each component model, using an existing variable for describing all the working modes of the components, defining the transfer conditions of all the working modes in each component, building modules, instantiating the components in the modules, and describing the structure of the system according to the connecting relations between the components. The system comprises a visual modeling module, a communication module, a model verification module, a scene input module, a diagnosis module and a result displaying module. Through the electrical system modeling method and the fault diagnosis system, the structural knowledge of the system, the functional knowledge of the components forming the system and previous experiential knowledge can be comprehensively utilized to carry out abnormity evaluation and fault location on the current system state, and the electrical system modeling method and the fault diagnosis system are used for improving the accuracy of diagnosis and the distinguishability of diagnosis results.