Abstract: As important equipment in power system, the trouble prevention and life management of transformer are of important significance for improving the reliability of power system operation. To implement the high-efficiency assessment on the fault state of transformer equipment, starting from the correlation and complexity characteristics of the transformer fault mechanism, a transformer fault state assessment model based on the coupling of polynary physical fields and fuzzy number theory was established. Firstly, based on the correlation characteristics of transformer faults, a network diagram of transformer fault evolution was established. Secondary, considering the complexity of transformer failure mechanism and limited monitoring data, the COMSOL software was utilized to carry out the coupling simulation of polynary physical field. Thirdly, combining with fuzzy number theory, an evolutionary probability calculation model for transformer faults was built. Finally, led in the risk entropy to characterize the uncertainty in transformer fault evolution, and the maximum probability expression of transformer fault evolution path was proposed and translated into a linear programming problem, and then the maximum probability fault evolution path of transformer under different initial risk factors was obtained. Results of analysis on computing example show that the calculation results by the built model can effectively reflect the feature of transformer fault statistic data, thus it can provide important reference for the state assessment of transformer.