Upper diameter data is difficult to obtain and the trunk segment boundaries are difficult to determine accurately by using traditional stem form index research method. To solve the above problems, a method based on stumpage non destructive accurate measurement technique by using the electronic theodolite was proposed. 198 larches were selected as samples and each one was divided into ten equal segments. Relative stem shape index of all segments were calculated. Firstly, significant differences between those relative segments were found by using variance analysis method. Subsequently, a tree could be aggregated with 2~10 trunk segments by using cluster analysis. In order to acquire the optimal quantity of segments for volume calculation, all statistical combination was tested by comparison of relative stem form index and volumes observed using electronic theodolites. The results showed that the optimal quantity of segment was obtained when the stem was divided into three segments, average relative stem form indexes for each segment (corresponding to height) were: 4.295（H0.0~H0.1）,1.524（H0.1~H0.4）,1.670（H0.4~H1.0），where H is the total height of a tree. Those parameters could be quantitatively described as trunk-shaped boundary. Thus a tree volume could be accurately measured by just measuring diameters at the corresponding height, which could effectively reduce the labor consumption on field survey and related researches.