The present invention relates to a network guard unit for an industrial embedded system and a guard method. The specific method is to form the network guard unit (NGU) through security technologies, such as integrated access control, identity authentication and communication data encryption, to provide active guard for a site control device. The NGU comprises an access control module, an identity authentication module, a data encryption module, a key negotiation module and a PCIE communication module, and supports the communication modes of dual network cards and PCIE bus. The present invention builds a secure and trusted operating environment for industrial control systems in combination with an active guard technical means in the field of information security on the basis of ensuring the correctness and the feasibility of security of various terminal devices in the industrial control systems.