A communication system for one-way transmission, comprising: a one-way link circuit comprising a programmable logic device; a grandmaster clock; a transmitting server in a secure site; a receiving server in an unsecure site, coupled to the one-way link circuit; and a precision time protocol switch coupled to the one-way link circuit, the grandmaster clock, and the transmitting server, wherein the precision time protocol switch is configured to receive a first synchronization message from the grandmaster clock, generate a second synchronization message according to the first synchronization message, and transmit the second synchronization message to the transmitting server, the programmable logic device, and the receiving server, wherein a time synchronization process among the transmitting server, the receiving server, and the programmable logic device is completed after the second synchronization message is transmitted; the transmitting server is configured to generate a timestamp according to the second synchronization message and transmit at least one data packet and the timestamp to the precision time protocol switch; the precision time protocol switch is configured to forward the at least one data packet and the timestamp to the programmable logic device; and the programmable logic device is configured to determine whether to output, to the receiving server, the at least one data packet according to the timestamp and the second synchronization message, wherein the receiving server is configured to receive the second synchronization message, the at least one data packet, and the timestamp from the programmable logic device, wherein the receiving server is configured to determine whether the at least one data packet is abnormal according to the timestamp and the second synchronization message and perform the time synchronization process with the precision time protocol switch according to the second synchronization message, and wherein the programmable logic device has a unidirectional mode and a bi-directional mode, wherein the precision time protocol switch is configured to communicate with the receiving server for the time synchronization process via the programmable logic device in the bi-directional mode, wherein the programmable logic device is configured to output the at least one data packet in the unidirectional mode. a one-way link circuit comprising a programmable logic device; a grandmaster clock; a transmitting server in a secure site; a receiving server in an unsecure site, coupled to the one-way link circuit; and a precision time protocol switch coupled to the one-way link circuit, the grandmaster clock, and the transmitting server, wherein the precision time protocol switch is configured to receive a first synchronization message from the grandmaster clock, generate a second synchronization message according to the first synchronization message, and transmit the second synchronization message to the transmitting server, the programmable logic device, and the receiving server, wherein a time synchronization process among the transmitting server, the receiving server, and the programmable logic device is completed after the second synchronization message is transmitted; the transmitting server is configured to generate a timestamp according to the second synchronization message and transmit at least one data packet and the timestamp to the precision time protocol switch; the precision time protocol switch is configured to forward the at least one data packet and the timestamp to the programmable logic device; and the programmable logic device is configured to determine whether to output, to the receiving server, the at least one data packet according to the timestamp and the second synchronization message, wherein the receiving server is configured to receive the second synchronization message, the at least one data packet, and the timestamp from the programmable logic device, wherein the receiving server is configured to determine whether the at least one data packet is abnormal according to the timestamp and the second synchronization message and perform the time synchronization process with the precision time protocol switch according to the second synchronization message, and wherein the programmable logic device has a unidirectional mode and a bi-directional mode, wherein the precision time protocol switch is configured to communicate with the receiving server for the time synchronization process via the programmable logic device in the bi-directional mode, wherein the programmable logic device is configured to output the at least one data packet in the unidirectional mode.