The present invention provides an OLED back plate and a manufacture method thereof. In the manufacture method of the OLED back plate of the present invention, by forming the planarization layer on the interlayer dielectric layer, and the planarization layer can serve as the mask of the etching process of the interlayer dielectric layer, and also can make the surface of the second source made on the surface thereof be flattened, which is advantageous to increase the area of the OLED light emitting area and to increase the aperture ratio. In the OLED back plate of the present invention, by forming the planarization layer on the interlayer dielectric layer, the surface of the second source made on the surface of the planarization layer is flattened, of which the OLED light emitting area is larger and the aperture ratio is higher.