High-brightness in-band pumping sources are used in tandem-pumping. It makes it possible to pump close to the emission wavelength, so the quantum defect heating can be low. This allows for the use of a shorter fiber. Therefore, it is easier to avoid thermal and nonlinear limits simultaneously. In view of the important of tandem-pumping in high power fiber laser, the experimental researches of tandem-pumped over seas are introduced firstly, including Yb-doped, Er-doped, Tm-doped, Ho-doped fiber lasers. Then the advantages such as restraining unwanted excess gain for higher order modes in cladding-pumped large-core Yb-doped fiber amplifiers are presented. And the potential benefits of reducing fiber photodarkening are also analyzed. At last, the problem and techniques of the tandem-pumping in high power fiber lasers are discussed.