The second harmonic susceptibility X/sup (2)/ (2.omega.) of a compositionally asymmetric coupled quantum well (CACQW) under the influence of the applied electric field are investigated theoretically. Analytical forms of the second harmonic susceptibility are derived by using the density matrix formalism. Coupled one-dimensional Schrodinger equation and Poisson equation are solved self- consisted to find subband eigenenergies and the envelope wave functions of the CACQW. Dipole moment elements for the CACQW are evaluated from the resulting envelope wave function and large nonlinear optical effects are predicated for this structure. By a suitable choice of the composition of CACQW, a novel structure which the second order nonlinear optical effect can be turn on or off by the applied electric field is proposed base on the results of the theoretical calculation. This phenomenon is attributed to the symmetry restoration of envelope wave functions of the CACQW under the quenching electric field. A simple physical model to estimate the quenching electric field has also been developed successfully.