The invention provides a method for calculating square wave carrying capacity of AC power cables. The method is characterized by comprising Fourier transform and temperature difference calculation caused by frequency doubling current, the size and period of square wave current can be reversely deduced from temperature constraint conditions of the cables through Fourier transform, and the method can be used for determining square wave carrying capacity in line transformation capacity expansion projects of the AC power cables, is scientific and reasonable, and has important practical significance and engineering value.