Tapered fiber is fabricated from traditional uniformed fiber using fused conical tapering, mechanical affection, chemical corrosion and so on, whose radius is various with the length of fiber. The changing part of fiber core is called tapered area. When the tapered area is distributed in whole double-clad fiber, the fiber is called a tapered double-clad fiber (T-DCF). The developing progress of tapered fiber is summarized, the advantages and basic theories of application of T-DCF in fiber laser area are described, some major experimental results of T-DCF are discussed, and the developing trend of T-DCF in fiber laser application is listed.