Quantum cascade laser (QCL) is a new-style laser whose wavelength remains in middle and far infrared. Hitherto its dynamic characters like pulse and modulation responses haven't been well acquainted. To ascertain this, an equivalent circuit model was derived on the base of rate equation of QCL that is invented by Bell Lab in 1994 by analyzing the unipolar transportation and transition behaviour of electrons in multiply quantum well, and the modulation response of QCL is gotten in the circuit simulation program PSPICE. Consequently some factors such as transition time between wells have been analyzed which could affect the modulation character of QCL.At last its dynamic character is found to be unexcellent compared with other lasers because of its peculiar energy band structure.