Aiming at transformer inter-turn short problem, this paper presents an idea which is equivalent the shorted winding to two different windings. The modeling of equivalence is proposed with the low voltage side inter-turn short of three windings as an example. First, the shorted three-winding transformer is equivalent to a four-winding transformer. Then a formula in the middle and low voltage sides is derived, according to which shorted information is calculated and the decrease of middle and low voltage sides is deduced and the latter decreases more seriously. The model is applied successfully to the simulation and analysis of the Guangxi power grid 220 kV transformer inter-turn short circuit by MATLAB, the simulation curve and result of analysis are coincide with examination results and recorded curve, which verify the correctness of the model.