Apllying the Bayesian network in the traffic network failure degree approach in a creative way.By using the method of empirical study,this paper does research on the traffic network failure degree approach based on Bayesian network with the evaluation index of connectivity reliability.The paper conclude the method of the traffic network failure degree quantification based on bayesian network by analysising the cause of traffic network failure, establishing the event tree and fault tree and then buliding the Bayesian network with the algorithm of transforming event tree and fault tree to Bayesian network.Finally, The paper cite a simple example of the road network, according to the above-mentioned method, establish the Bayesian network of road network failure and use the software MSNBX to simulate the different types of attacks, then get the result to prove the validity of the method that had been mentioned in the paper and give some advise to the application of bayesian network in research on large scale traffic network.