In the instruction of computer courses, some students copy other’s source code as themselves. In order to detect this plagiarism accurately, researchers did a lot. In this paper, we described a source code plagiarism detection technologe based on AST, This technologe can detect the plagiarism accurately when the position of functions is changed by plagiarist. At first, transforming the programs to the AST using ANTLR, and then, abstracting the function subtrees from the AST, at last, compare the function subtree using LCS, get the simalarity between programs.