Nowadays Computer device and Internet are improving rapidly.People's living standard will be better and better,so expertise will be more important day by day.However,decisions are possible to be changed by experts because of some factors such as experts' feeling,experience,age and so on.Therefore,people can't work 24 hours to solve the other people's problems as machine.Facing this situation,that's the reason why we want to study the feature and structure of Expert System to help people solve their problems fast and conveniently. In the study,we try to combine Expert System and Decision Tree and create a system that can deal with complicated and high repeatability problems.Thus,we can infer and make proper decision without experts by using Inference Engine and Decision Tree.