A transmission system and a transmission method for network address translation traversal are provided. The transmission system includes a private network device, a network address record device, a public network device and a network address translation server. The network address record device records an inner network address of the private network device and an outer network address corresponding to the inner network address. The public network device inquires the inner network address of the private network device and the outer network address corresponding to the inner network address from the network address record device, and generates a packet according to the inner network address and the outer network address corresponding to the inner network address. The network address translation server receives the packet from the public network device, and transmits the packet to the public network device.