Tunable external cavity diode laser(ECDL)has excellent characteristics such as narrow linewidth, wide range tuning, single-mode output and high output power. Tunable ECDL is widely used in white light interferometry, wavelength division multiplexing system, coherent optical communication, optical fiber sensor technology, etc. In this paper, the basic principles of tunable ECDL and the tuning mechanisms of various typical external cavity structures are introduced firstly. Then the latest domestic and overseas research progresses of various tunable ECDL are expounded according to different external cavity structures, and the advantages and disadvantages are also analyzed. Finally, the existing shortcomings and future development trends of tunable ECDL are summarized and prospected.