Based on the group theory and the complex division method, the dimension classified method for planar multi-loop mechanism was analyzed. Firstly, the dimension classified inequation was derived from the singularity of constraint equation of the mechanism. Subsequently, by using the classified inequation, the complex division method of the structure-parameter-space was explored. By using the mechanism symmetry group, the simplification method of the division results was analyzed. Finally, by using an example, the classified methods were validated. The results show that all information about mechanism classification are included in the division results. Besides, this method is appropriate for complicated mechanism classification by computer.