In the past few years, more and more fields have realized the values and potentials of digital twin. Therefore, many research institutes have carried out the exploration and practice on digital twin. To better master the research worldwide, the global academic research of digital twin is investigated at first, and a comparative analysis of digital twin research in different countries is also given. Furthermore, the various applications of digital twin are summarized, including smart manufacturing, city management, healthcare, aerospace, etc. Moreover, the five-dimension digital twin model is introduced, as well as its applications in ten industrial fields. At last, the related works of Digital Twin Research Group at Beihang University are briefly introduced.