Topology structure design of low-DOF parallel stable platform was studied based on POC set theory and SOC unit. Taking the 1-translation and 2-rotation (1T-2R) parallel mechanism as an example, the detail procedure for low-DOF parallel stable platform topology structure design was formulated. The SOC branches with 1T-2R output were presented, and many new 1T-2R topology structure types were obtained by arranging the suitable SOC branches. The design personnel may select the best topology structure from these obtained structure types and its topology parameters based on concrete performance indices and other design requirements. And the method has some reference for other low-DOF stable platforms or PM structure designing and election.