Abstract: The communication over spanned network under the network isolation environment, requires encryption protection for the transmitted information. The existing security solutions, whether static encryption or dynamic encryption, may be cracked and forged. To solve this problem, we design and implement a dynamic encryption transmission method across gateway using the timestamp mechanism. This method was based on the time element and realized the random change of the key through the change of time. It is analyzed and verified from three aspects of randomness, security and consistency, and the results show that the method not only realizes the dynamic randomness of data encryption, but also ensures the consistency of data decryption over spanned network, and can resist cryptanalysis attacks to a certain extent.