There are three classes of viral fusion protein. Different types of viral fusion proteins usually have radically different architectures, whereas they share similar “hairpin” conformation during fusion processes. After triggering, a hydrophobic domain, fusion loop or fusion peptide insert into target membrane and the opposing membrane were brought closely with fusion protein folding and hairpin formation. With the energy from viral fusion protein conformational changes, the membranes were broken and fusion together. Here, we reviewed the characteristics of three classes viral fusion proteins and the functions of central trimer-of-hairpin domain, transmembrane domain and juxtamembrane domain in fusion process.