The invention discloses a covert communication method based on IPID (Internet Protocol Identification) bit delta modulation. By using the method, sensitive information to be transmitted can be concealed in a head part of a network data packet in a computer network environment, so that the sensitive information can not be easily perceived. The method comprises the following steps: when a transmitting party transmits a network data packet, embedding covert information into low-order 4 bits of the IPID according to the corresponding modulation method, wherein high-order 8 bits of the IPID are used for implementing the incremental mode so as to enhance the covertness, and high-order 4 bits of low-order 8 bits are used for implementing synchronization and verification of data communication; and after a receiving party receives the data packet, extracting data of the low-order 8 bits, wherein the high-order 4 bits are used for verification, and the low-order 4 bits are used for the covertness of the data. The method utilizes the self incremental mode during normal communication of the IPID bits, has the advantages of favorable covertness and attack resistance, and is suitable for transmitting sensitive information with a small data size.