IHEP (the Institute of High Energy Physics, Beijing, China) has started the R&D of high field accelerator magnet technology from 2014 for recently proposed CEPC-SppC (Circular Electron Positron Collider, Super proton proton Collider) project. The conceptual design study of a 20-T dipole magnet is ongoing with the common coil configuration, and a 12-T model magnet will be fabricated in the next two years. A 3-step R&D process has been proposed to realize this 12-T common-coil model magnet: first, a 12-T subscale magnet will be fabricated with Nb3Sn and NbTi superconductors to investigate the fabrication process and characteristics of Nb3Sn coils, then a 12-T subscale magnet will be fabricated with only Nb3Sn superconductors to test the stress management method and quench protection method of Nb3Sn coils; the final step is fabricating the 12-T common-coil dipole magnet with HTS (YBCO) and Nb3Sn superconductors to test the field optimization method of the HTS and Nb3Sn coils. The characteristics of these R&D steps will be introduced in the paper.