In this paper,a novel method is introd uced to optimize the active layer structure of the THz quantum cascade lasers (QCLs) with a multi－quantum well material,and the influence of inje ctor barrier thickness in device active layer on device performance is mainly studied by a novel equivalent circu it model.Firstly,transport characteristic parameters such as the nonradiative scattering times,radiative spontaneous relaxation time and electron escape time are obtained by a self－consistent scheme,and then an equi valent circuit model of THz QCLs is introduced by revising three level multi－mode rate equation s.Two critical model parameters of electron spontaneous emitting time and photon lifetime in the laser cavity are also prese nted.Finally,depending on the equivalent circuit model,the photoelectric responses of THz QCLs with the thickn ess of injector barrier from 3.0nm to 6.8nm are obtained by using a general circuit simulator PSPICE,and the effects of injector barrier thickness on device performance parameters such as threshold current,output opt ical power and input impedance are also discussed.Results indicate that the injector barrier thickness is a cr itical parameter for improving the device performance.