The invention discloses a single chip extra-large data external memory extension system and a control method thereof. The system comprises a single chip, a plurality of first decoders, a plurality of extended I/O port chips, a plurality of second decoders and a plurality of memories, wherein each of the memories is connected with address, data and control buses of the single chip; an input end of each of the first decoders is connected with the address bus of the single chip and an output end of each of the first decoders is connected with an enabling end of each of the extended I/O port chips; an input end of each of the extended I/O port chips is connected with the data bus of the single chip; the extended I/O port chips correspond to the second decoders one to one; an input end of each of the second decoders is connected with an output end of the corresponding extended I/O port chip; an output end of each of the second decoders is connected with a chip selection port of each of the memories. According to the single chip extra-large data external memory extension system and the control method thereof, the capacity limit of the traditional extension method is broken through, and the real-time access demand of ultra-large capacity is satisfied.