This paper is based on an online handwritten mathematical expression recognition system. In this paper,we propose a structural analysis approach for mathematical expressions based on the partitioned tree transformation approaches. The approach firstly divides the mathematical expression into several subexpressions ,according to its internal structural information . Then,with a tree transformation it displays the spatial relations of all the characters of each subexpression. In this algorithm,the spatial relation types are greatly simplified,and the algorithm can deal with the expression with a nested structure .Moreover,it can be easily enhanced to suit for different kinds of applications.