Abstract: In order to precisely describe the information flow from 1E degree to non 1E degree subsets of the network in the Nuclear Power Plant network, a new security information flow mathematics model based on this network is given in this paper. This model incorporated the new concept of the planar-classification function, which combined the classification of the sub-network and the classification of object. Safety classification of the objects was divided based on the function values and the information flow policy is described with the function values of two objects. Strict mathematic verification showed that this information flow model is reasonable and safe, which can reflect the safety requirements of the Nuclear Power Plant network.