Back conversion is the key factor that affects the conversion efficiency, the beam quality of the parametric waves, and the stability of the output in the optical parametric process. For the reason of the variation of the back conversion with the distribution in the cross-section and with time, the back conversion cannot be eliminated thoroughly. We analyzed the problem of the back conversion in the optical parametric process, and established the models of the parametric process for the continuous waves and the pulsed waves, which help us study the key factors affecting the back conversion. According to the conclusion of our analysis, the proper length of the nonlinear crystals, the optimal distribution of the energy of the pump beam, and the proper parameters of the resonant cavity ( for optical parametric oscillators ) could reduce the back conversion and help to increase the conversion efficiency. According to the theoretical results, we designed an X-cut KTA (Potassium Titanyl Arsenate) optical parametric oscillator, and obtained the signal light with the energy of 270 mJ and the idler light with the energy of 150 mJ, which decreased the back conversion, and the efficiency reached 43%.