The invention relates to an extra-high voltage alternating current and direct current hybrid cross-grid power transmission system and method. The system comprises extra-high voltage alternating current power transmission equipment, extra-high voltage direct current power transmission equipment and a receiving-end grid scheduling center, wherein two ends of the extra-high voltage alternating current power transmission equipment are respectively connected with a sending-end grid and a receiving-end grid; the extra-high voltage alternating current power transmission equipment is used for transmitting power in a form of 1000 kv alternating current; two ends of the extra-high voltage direct current power transmission equipment are respectively connected with the sending-end grid and the receiving-end grid; the extra-high voltage direct current power transmission equipment is used for transmitting power in a form of 800 kv direct current; the receiving-end grid scheduling center is connected with each of the extra-high voltage alternating current power transmission equipment and the extra-high voltage direct current power transmission equipment and used for controlling the power input sent to the receiving-end grid by the sending-end grid through the extra-high voltage alternating current power transmission equipment and the extra-high voltage direct current power transmission equipment. Compared with the prior art, the extra-high voltage alternating current and direct current hybrid cross-grid power transmission system and method adopt an extra-high voltage alternating current and direct current hybrid power transmission manner; by adopting 1000 kv alternating current and 800 kv direct current to transmit power, electric loss in the power transmission process can be greatly decreased relative to the power transmission manner of 500 kv alternating current, so that favorable economic benefits are provided.