In order to obtain as high efficiency of difference frequency conversion as possible, a method based on the cascaded electro-optic and difference frequency theory in the quasi-periodically poled LiNbO3 optical superlattice (QPPLN) is put forward. The idea is that an external electric field is applied along the y-axis of the QPPLN to control the energy conversion among the pump beam, signal beam, o-polarized difference-frequency beam and e-polarized difference-frequency beams. The numerical results show that, in a 40-mm long QPPLN at 100 ℃, when the ratio r of the intensity of the 1550-nm signal beam to that of the 1064 nm pump one, is less than 0.324, any pump light with an intensity higher than a specific value can be fully translated into the 1550-nm signal light and the 3393.4-nm difference-frequency light as long as a suitable electric field is applied. In the case that r is equal to or larger than 0.324, only in a fixed range can the pump light be translated into signal and difference-frequency ones completely under an appropriate applied electric field. Out of the range the applied electric field cannot increase the efficiency of difference-frequency conversion. The further investigation indicates that the difference-frequency conversion modulated by electro-optical effect is not sensitive to both temperature and domain structure error.