The invention provides an alternating current transmission system. The alternating current transmission system comprises a main generator, a variable flow module and an excitation module, wherein the main generator is used for generating and outputting alternating current; the variable flow module is used for conducting variable flow processing to the alternating current generated by the main generator, outputting the alternating current which fits in with a traction motor nominal standard and is obtained after being processed to a traction motor and outputting the alternating current which fits in with an auxiliary assembly nominal standard and is obtained after being processed to an auxiliary assembly; the excitation module is used for receiving the alternating current output by the main generator, converting the alternating current into exciting current and outputting the exciting current to the main generator; the output power of the main generator is controlled through the exciting current.