According to the problem of dissolved gas analysis’ improved three-ratio method which fault area boundary value fuzzy judgment in transformer fault diagnosis, with the analysis of improved three-ratio method, using Euclidean distance to represent probability of each failure to establish the basic belief assignment(BBA) diagnosis model of different failure for transformer fault diagnosis. And with the D-S synthetic rule fuses the BBA of different fault, realizing the function of reconstruction and normalization the multiple BBA model. By using the BBA model to calculation for a transformer fault of 500kVA substation in changchun, and through the improved three-ratio method to contrast and analyze. Results show that the model can diagnose a variety of transformer faults accurately and effectively; the form of probability makes up the defect of improved three-ratio method that boundary value calculation fuzzy, which makes the criterion of failure is more truthful and accurate. The BBA model can provide an effective method for solve the power transformer fault diagnosis.