In this work, the modeling strategy of quantum-cascade lasers is generalized to including the harmonicresonance-enhanced nonlinear oscillations up to the third order. In addition to the lasing power at fundamental frequency, the second-harmonic and third-harmonic output power can all be evaluated through the model. The model is built on the rate-equations for subband electrons and photons, in connection with Maxwell wave equations for propagation coupling between different emission modes. Various radiative and nonradiative carrier scattering mechanisms are accounted in the model. In addition to single-photon processes, multi-photon processes in the nonlinear cascades are also accounted. The model is based on a full cascade structure containing the injector, active region, and collector in order to account for the nonideal injection efficiency between periodic stages of quantumcascade lasers. The simulation results on two fabricated quantum-cascade structures capable of nonlinearly harmonic generations agree well with the experiment measurements. The model can be used to evaluate and further optimize the nonlinear performances of quantum-cascade lasers.