It is difficult to detect and find fault for traditional protection when transformer short circuit fault occurs at inter-turn, especially when the turns are small. This paper proposes a new method using variation of current ratio of two winding sides of transformer, based on the corresponding relations between the current ratio. The protective action criterion is constructed. The new method not only avoids the influence of the internal phase fault and the external short-circuit, but also distinguishes the inter-turn short circuit from other faults with a high sensitivity. The validity of this method has been proved by tests on a simulation transformer.