Network covert channel is the information channel that carries on covert information transmission in violation of the communication restriction rules under the network environment. It brings new challenges to the network information security and provides new research point for ensuring the security and privacy of data transmission. Firstly, the basic concepts of network covert channel are introduced, such as definition, classification, capability dimension. Then, network covert storage channel and network covert timing channel construction technologies are sorted out from three aspects of symbol design, information coding and channel optimization. Then the evaluation methods of network covert channel are summarized from three aspects of covertness, robustness, and transmission efficiency. Furthermore, the countermeasure technology of network covert channel is sorted from three aspects of elimination, restriction, and detection. Finally, some future research directions are prospected.