The invention relates to a method for protecting a multi-phase autotransformer for an aircraft, including the steps of receiving (100) values of current output from (Ia, Ib) the first and second phases and a value of current input into (IB) the second phase; determining (101), from these received values of the second phase, a value of current representative of the operation (IFb) of the second phase; determining (102), as a function of this determined value and of the value of current output from (Ia) the first phase, a value representative of the homopolar current (Idet-a) flowing in the first phase; comparing (104) this value of homopolar current (Idet-a) with a first predetermined threshold value (S1) at least during a first predetermined period (D1); and controlling (106) the values of currents input into (IA, IB) and/or output from (Ia, Ib) the phases as a function of the said first comparison.