The present invention discloses an epitaxial structure and a preparation method for a grating structure thereof. The epitaxial structure comprises a substrate and a waveguide layer laminated along a first direction on the substrate. In a second direction perpendicular to the first direction, the waveguide layer is divided into quantum well intermixing enhancement regions and quantum well intermixing suppression regions. The quantum well intermixing enhancement regions and the quantum well intermixing suppression regions are alternately arranged in a region in the second direction, so that a grating structure is formed due to that a refractive index of the quantum well intermixing enhancement regions and the quantum well intermixing suppression regions changes periodically in the second direction. According to the epitaxial structure and the preparation method for the grating structure thereof, the alternately arranged quantum well intermixing enhancement regions and the quantum well intermixing suppression regions form the grating structure, so as to avoid using a complex secondary epitaxy manufacturing process, thereby reducing the manufacturing cost; the effect of the secondary epitaxy manufacturing process on reliability and performance of a device is avoided; and a grating period can be regulated in a convenient and effective way.