Abstract: The unsaturated seepage mechanism in a rock fracture is discussed. It shows that the macroscale(fracture scale) unsaturated seepage can be considered as many microscale(local aperture scale) saturated seepage. Then the critical seepage characteristics are studied by means of renormalization group(RG) theory while the percolation happens. The results show that the critical saturation does exist and there is not any seepage flow.if is less than the critical saturation. Finally, the unsaturated hydraulic conductivity is also discussed.