Aiming at the isolation degree in distribution communication network based on PTN technology, this paper proposes a mathematical model to evaluate the effect of isolation in distribution communication network. The feature of mutual information, together with the change frequency generated by the influence of Y on X, is used to evaluate the correlation between channels, and the isolation degree is evaluated by the size of mutual information. This paper uses this method to calculate and compare the isolation degree of the PTN technology with that of layer 2 switch technology by simulating the test network architecture diagram of Yunnan Branch of China Southern Power Grid. The experiment results show that this method is feasible and effective, which provides a quantifiable method for evaluating the network isolation.