The technical performance of modern cars is becoming better and better, and the automobile gearbox plays an important role in which. For instance, the use of a Continuously Variable Transmission (CVT), an automatic transmission that can change seamlessly through a continuous range of effective gear ratios, makes the driving more comfortable and consistent. But it is also disadvantageous in that the transmission belt of the CVT is easily damaged. In order to diagnose CVT faults in time and effectively, data mining technology is introduced in. Decision tree classifier is a kind of classification algorithm based on examples, which is widely used all over the world. This paper is based on the data set of the fault diagnosis of CVT, and the ID3 algorithm is used to build the decision tree. Then the rules are extracted to help judge the fault diagnosis, so as to ensure the normal operation of CVT.