The utility model provides a square wave alternating-current power transmission system for improving alternating-current cable line transmission capacity. The square wave alternating-current power transmission system includes a sending-end voltage source converter, a sending-end interpolar power coordination control device, a receiving-end interpolar power coordination control device; the output end of the sending-end voltage source converter is connected with the sending-end interpolar power coordination control device; the sending-end interpolar power coordination control device is connected with the receiving-end interpolar power coordination control device through an alternating-current cable line; and the receiving-end interpolar power coordination control device is connected with a receiving-end alternating-current system through the receiving-end voltage source converter. According to the square wave alternating-current power transmission system of the utility model, excellent control performance of the modular multilevel voltage source converters are utilized, and interpolar voltage change-over switches and interpolar current change-over switches are used in cooperation, and therefore, problems existing in capacity increasing transformation of the alternating-current cable line can be solved, and influence of space charge accumulation on the insulation of the cable line can be decreased. The square wave alternating-current power transmission system has functions such as dynamic reactive power compensation and harmonic suppression. Thus, the square wave alternating-current power transmission system is of great significance for reconciling contradiction between increasingly growing load and increasingly difficult new line construction.