Nonlinear optics of low-dimensional semiconductor structures has recently attracted considerable attention. The problem is very important to the development of super-lattice devices such as fast infrared detectors and modulators because one needs to know the range of input optical intensities for which linear operation is possible. Analytical expression of the second-harmonic generation (SHG) coefficients is obtained in the Poschl-Teller well using the density matrix formalism taking into account the intrasubband relaxation. Since there are two adjustable parameters K and A in the Poschl-Teller well, the shape of the well will change with the change of the parameters K and A, accordingly the SHG coefficient will change with the parameters K and A. It is shown that the SHG coefficient depends on the parameter K , A and the tunnel bandwith. Numerical results are presented for a typical AlGaAs/GaAs Poschl-Teller well.