This paper mainly introduced the research on constructing Mongolian Treebank based on phrase structure grammar. Having Considered related Mongolian Treebank work and Mongolian words characteristics, we developed a Mongolian syntactic tagset. The tagset includes two kinds of tags. One is syntactic constituent tag and the other is grammatical relation tag. On the basis of the tagset, we developed the Mongolian Treebank auxiliary processing system. Finally, we built a Treebank that contains 3645 sentences and did an experiment on this Treebank.