Abstract: The stable and instantaneous theory of the optical frequency mixing in BBO crystal are studied. The influences of the pump parameters (including the wave length, the intensity, the pulse width, the line width and the walk-off angle of the pump light) on the conversion efficiency, pulse width and the optimum length of the nonlinear crystal of the optical frequency mixing are discussed. The results show that if the quality of pump light is improved, the crystal length is optimum, the pump intensity, wave length and pulse widtht are suitable, the conversion efficiency will be much higher.