In order to increase the second harmonic generation conversion efficiency, the periodic optical superlattice was sandwiched with nonpoled and nonlinear media with finite and variable width, which can bring about the significant modulation for the fundamental wave with changing the thickness of the additional media. It is found that: the conversion efficiency increases or reduces when the energy of fundamental wave increases or reduces; it can be enhanced significantly when transmission resonance (the rate of transmission obtains 1) takes place; compared with the conventional sample (the optical superlattice embedded in infinite and nonlinear media), the second harmonic generation conversion efficiency is enhanced 1.5 times at most; for the practical sample (the optical superlattice embedded in air), it can be enhanced 3.3 times at most. So the second harmonic generation conversion efficiency can be enhanced apparently by the transmission resonance.