A method for preventing NAT-PT device from attack includes setting NAPT table and content field lf limitation table then initializing them, setting maximum value Max and minimum value Min of NAT-PT device for allowing certain host to set up link item, counting up items in NAPT table and link item number of certain host, deciding whether certain host is listed in limitation table, or not according to counted up result and confirming data packet processing flow to link sum in limitation table, calling update operation of NAPT table and limitation table on fixed time and updating link sum .