In this paper,we study the influence of incoherent irradiation on the conversion process from self-defocusing to self-focusing in LiNbO3:Fe crystal under the open-circuit condition.We observe that the incoherent irradiation can increase the refractive index change, fasten greatly the process of self-defocuisng, self-focusing and the conversion from the former to the latter in LiNbO3：Fe crystal, whether it propagates along the direction of the incident light or the opposite direction. When the incoherent light illuminates the crystal in certain time intervals repeatedly, the transmissivity can rise up and fall down quickly, the obvicus switching effect is observed. The change of the refractive index controlled by the incoherent irradiation can be applied in the all-optical switch.