Abstract: As the heaviest elementary particle discovered so far, the top quark plays an important role in the Standard Model of particle physics and the search for New Physics. The stability of the electro-weak vacuum is very sensitive to the top quark mass. However, the top quark mass cannot be measured directly due to its short lifetime. This means that its experimental measured value depends on the theoretical definitions. In this talk, I will explain why the renormalon effect causes the inherent ambiguity (about 200 MeV) of the most widely used pole mass. Then I will introduce several unambiguous definitions of short-distance masses, such as the potential subtracted mass, the 1S mass, and the MS-bar mass. In the end I will discuss how to measure the top quark mass accurately at the future Circular Electron Positron Collider (CEPC).