Based on the singularity of constraint equations and the division of dimension space of links, the classification method of spherical single-loop mechanisms was studied. Firstly, the classification conditions were derived from the singularity of constraint equations. Secondly, the complex division process of dimension space of links was investigated. Finally, the classification method was validated by using 4-, 5- and 6-link spherical mechanisms. It is shown that the classification method is comprehensive and all the classification information is included in the complex division results, and this method is appropriate for the classification of complicated mechanisms by computers.