Most of the existing diagnosis systems used in distribution network are expert systems based on experiences， which is not able to diagnose the faults beyond experience and is very difficult for system transplanting and maintaining. In order to overcome the defect of the traditional expert system， the method of model-based diagnosis（MBD） is applied to the distribution network. Through the introduction of a few basic concepts， a scheme using MBD in the fault diagnosis of distribution system line is proposed. Finally， an example of actual distribution network is adopted to verify the feasibility and validity of the method through modeling， programming and testing.