The disclosure relates to a polycrystalline diamond composite sheet having a ripple-shaped gradient layer and a preparation method thereof. The polycrystalline diamond composite sheet consists of a cemented carbide substrate, a ripple-shaped gradient layer of a multi-layer structure, and a polycrystalline diamond layer from bottom to top. In the ripple-shaped gradient layer, a content of polycrystalline diamond increases from bottom to top, and a content of cemented carbide decreases from bottom to top. In the ripple-shaped gradient layer, an amplitude of a ripple-shaped structure is 0.2 to 0.6 mm, a wavelength is 1 to 2 mm, a spacing between an upper ripple and a lower ripple of a top layer is set to a gradient of (t/2 to t) mm to t mm from a peak to a trough, and spacings between an upper ripple and a lower ripple of remaining layers are all t mm, wherein t is 0.05 to 0.4.