The space atomic clock, a beautiful piece of “watch” in Tiangong two, looks up at the deep space, overlooks the earth, explores the vast universe following the golden rule, and gives a detailed description of every inch of the earth mother. It is the first space atomic clock to carry out scientific research of the space laboratory in the world, its precision is also unprecedented with error less than one second in thirty million years. This atomic clock is gestated and developed by Chinese scientists taking a dozen years, which carries our positioning, deep space exploration, verification of general relativity and other research assignments.