A new method is presented to enhance the image edge using photorefractive two wave coupling. In the setup for real time writing and reading, the Fourier transform hologram using Cr:SBN crystal as the recording medium, the spatially separated high and low frequency components of the object beam will couple with the reference beam individually. The coupling results in the enhancement of the relatively weak high frequency component, because the intensity of the high frequency carrying the edge information of the object is much weaker than that of the low frequency. It discussed that the influences of the crystal axis orientation and the intensity ratio of the reference beam to object beam on the image edge enhancement. The physical mechanism of the image edge enhancement under different reference object ratio is analyzed. This Cr:SBN crystal can be used as a model of optical dynamic filtering for image edge enhancement.