The invention provides a GIS (Gas Insulated Switchgear) evaluation method which is implemented on a GIS, wherein the GIS comprises a plurality of subsystems and every subsystem comprises a plurality of elements. The GIS evaluation method comprises extracting historical data of the GIS and obtaining the fault rate of every element arranged in the corresponding subsystem according to a proportional hazard model; calculating probability values and evaluation values of every subsystem according to a preset algorithm, wherein the probability values are corresponding to the healthy state, the sub-healthy state and unhealthy state respectively and every probability value is correspondingly combined into the corresponding evaluation value; obtaining the defect and fault state of every subsystem to revise the evaluation values of every subsystem; establishing a bayesian network according to the GIS logical relation; obtaining the conditional probability of the GIS through the bayesian formula according to the bayesian network and the revised evaluation values to determine the system reliability. According to the GIS evaluation method and device, multiple information sources can be integrated, the complexity of the GIS can be combined, the numerical value of the reliability of the GIS can be quantized, and the reliability of the GIS can be accurately and rapidly evaluated.