We present a self-consistent approach to simulate the output characteristics of a quantum cascade laser (QCL), such as the current-light (I-L) and current-voltage (I-V) curves. Unlike the conventional 1(1/2) period model, our new QCL model includes the spontaneous emission and the stimulated emission, which allow the numerical study of the laser behavior above its threshold. The corresponding numerical method is proposed to solve the full laser rate equations in a self-consistent way. For a given QCL, the I-L and I-V curves are calculated, showing a good agreement with the typical QCL output characteristics. Besides the output properties, this new model also provides a way to study the photon and electron dynamics in the QCL cavity above the laser threshold. An example of the QCL photon and electron response to the interband ultrafast optical excitation is given at the end of this paper. All simulation results show the validity of the new model.