Nonlinear optical rectification in a special asymmetric parabolic quantum well is studied by density-matrix methods, and the analytical expression of optical rectification coefficient is derived. Finally, the numerical results are presented for a typical GaAs/AlGaAs asymmetric quantum well. The variation rules between rectification coefficient and the asymmetry of quantum well, the incident photon energy are obtained. The larger rectification coefficient was obtained in this special quantum well. This is a feasible way to get fine nonlinear materials in experiment.