A communication system and a communication method for reporting a compromised state in one-way transmission are provided. The communication method includes: receiving a packet by a first port; coupling an error checking circuit to the first port, wherein the error checking circuit checks a header of the packet; coupling a first unidirectional coupler to the first port and the error checking circuit, and coupling a second unidirectional coupler to the first port and the error checking circuit; in response to an error being in the header, disabling the first unidirectional coupler and the data inspection circuit and enabling the second unidirectional coupler by the error checking circuit; receiving the packet from the communication device by a receiving server; and in response to determining the received packet is incomplete by the receiving server, outputting the compromised state by the receiving server.