1. A pre-fetch control method used in a system having a cache controller, a pre-fetch controller and an instruction pre-fetch buffer, the pre-fetch control method comprising the steps of: receiving a data request for M-bytes requested data from the cache controller; determining whether the M-bytes requested data are found in the instruction pre-fetch buffer, and transmitting the M-bytes requested data to the cache controller if the M-bytes requested data are found in the instruction pre-fetch buffer; determining whether a combined access control is matched when the M-bytes requested data are not found in the instruction pre-fetch buffer; sending out a data request to an external unit for the M-bytes requested data if the combined access control is not matched and transmitting the M-bytes requested data returned from the external unit to the cache controller; and sending out a data request to the external unit for the M-bytes requested data and n*M-bytes extra data (n≧1) if the combined access control is enable, transmitting the M-bytes requested data returned from the external unit to the cache controller and storing the n*M-bytes extra data returned from the external unit in the pre-fetch buffer.