The invention provides a loss calculation method for an AC power cable under square wave current carrying. The method is characterized by comprising the following steps: calculating temperature difference and cable loss caused by fourier transform and frequency multiplication current; expanding square wave current into overlapped series sinusoidal current through the fourier transform; performing back calculation to obtain the loss of the cable caused by each sinusoidal current by adopting the cable AC current carrying quantity formula; and then performing the method of algebraic sum to obtain the loss of the cable under the square wave current carrying. The method is scientific, reasonable and high in practical value, and can accurately analyze the loss of the AC power cable under the square wave current carrying.