In this paper we will give an introduction to Circular Electron Positron Collider (CEPC). The scientific background, physics goal, the collider design requirements and the conceptual design principle of CEPC are described. On CEPC accelerator, the optimization of parameter designs for CEPC with different energies, machine lengthes, single ring and crab-waist collision partial double ring options, etc. have been discussed systematically. The sub-systems of CEPC, such as collider main ring, booster, electron positron injector, etc. have been introduced. The detector and MDI design have been briefly mentioned. Finally, the optimization design of Super Proton-Proton Collider (SPPC), its energy and luminosity potentials, in the same tunnel of CEPC are also discussed.