Many ways and methods can be used to process the complex problems,such as divide and conquer,which is an important problemsolving technique. The key technology during the decomposition is the problem of information preservation. A method of decomposing a single Bayesian network is proposed based on the conception and properties of the Bayesian network junction tree, and it can process the complex Bayesian networks effectively, and can also solve the problem of information preservation well as decomposing a Bayesian network. Because no conditional independency information is lost and no extraneous conditional independency information is introduced during the decomposition, this method is lossless. Finally, the paper detailedly analyzes the process of decomposing a Bayesian network with the Asia Bayesian network, and the effectiveness of this method has been verified by this example.