A method for diagnosing faults in products of one type includes determining a fault type of the products and all of the underlying reasons for the fault which correspond to the fault type. A probability of each underlying reason is calculated and a decision tree is constructed, a root node of the decision tree being the determined fault type and all the underlying reasons being child nodes of the root node. When the decision tree is solved, the underlying reasons for the fault of that type of product can be presented.