The invention provides an IPv4/IPv6 (Internet Protocol version 4/Internet Protocol version 6) conversion system and method based on an SDN (Software Defined Network). In the system, an IVI (Interchangeable Virtual Instrument) server is used for converting an IPv4 data packet received from an SDN switchboard into an IPv6 data packet and then returning the IPv6 data packet back to the SDN switchboard; the SDN switchboard is used for receiving the IPv4 data packet from a client and the SDN switchboard is used for judging whether the IVI server is normally operated or not; if the IVI server is operated normally, the SDN is used for transmitting the IPv4 data packet received from an IPv4/IPv6 dual-stack client to the IVI server and sending out the IPv6 data packet returned back by the IVI server through an IPv6 route; when the IVI server is operated abnormally, the SDN switchboard is used for transmitting the IPv4 data packet received from the client through a reserved IPv4 route.