A network system based on a SDN with a seamless handover mechanism, an operation method and a control apparatus thereof are provided. The network system includes network devices, host devices and a control apparatus. The network devices are connected together. A part of the network devices are equipped with wireless access networks of the same SSID, and each of the host devices is connected to one of the network devices. The control apparatus is connected to the network devices. When a first host device of the host devices is changed to connect with a second network device from a first network device, the control apparatus controls the network devices to return an address resolution response packet to the first host device in response to an address resolution request packet transmitted from the first host device, so as to maintain an IP address of the first host device.