The invention discloses an expansion system and a method for 256-bit keys based on an advanced encryption standard (AES), and mainly solves the problems of low efficiency and high power consumption in a key expansion process in the existing 256-bit AES. The process includes storing an initial key in the first round of key expansion, taking the first four rows as a round key of the key expansion in the first round, and performing word cycle, byte replacement, and bitwise XOR operations; executing a new byte replacement and a secondary bitwise XOR operation according to serial number n; taking results of the two bitwise XOR operations as a round key and storing in a local register and an external storage cell for the usage of encryption process reading; and repeating the operations for round keys obtained in a former round in following rounds until 14 round keys are obtained, and finishing key expansion. According to the expansion system and the method for the 256-bit keys, instantaneity of key expansion and reusability of round keys are achieved simultaneously, key expansion with high efficiency and low power consumption is achieved, and the expansion system and the method are suitable for key expansion processes of the 256-bit AES.