Nowadays,more and more colleges tend to organize online programming examination,which is benefit to improve students practice skills.However,this may bring a problem,as the programming source code is easy to copy,some students may cheat in the examination.The traditional manual method of cheating detection has some disadvantages,such as time-consuming and inaccuracy.To address this issue,an efficient and accurate method based on the Self-organizing maps (SOM) and tree structure similarity is proposed in this paper.We use abstract syntax trees to represent source codes.By SOM,all source codes are divided into several classes,and the source codes in each class are similar.And then we can detect cheating source codes in each class by using tree structure similarity.