Quantum Cascade Lasers(QCL) are the important laser sources in mid-infrared bands. The tunable mid-infrared quantum cascade laser has been a research focus because of tunable frequency and single longitudinal mode. At present, the tunable frequency of mid-infrared quantum cascade lasers can be realized by the Distributed Feed Back(DFB) grating, the Distributed Bragg Reflect(DBR) grating, the external cavity diffraction grating, etc. The principle and applications of mid-infrared quantum cascade laser are introduced. The progresses of the DFB and DBR tunable quantum cascade lasers, of the external cavity tunable quantum cascade lasers nearly 10 years are summarized. The advantages and disadvantages of various tunable methods are discussed. Finally, the development trends of tunable quantum cascade lasers are prospected.