The invention relates to the technical field of photoelectrons and provides a sub-wavelength grating reflection enhanced silicon-based wide-spectrum integrated optical detector and a preparation method thereof. The optical detector comprises a silicon substrate layer, a silicon oxide substrate layer, a sub-wavelength grating layer, a resin layer, an n-type epitaxial layer, an intrinsic layer and a p-type epitaxial layer which are sequentially formed from bottom to top, and the optical detector further comprises n-type contact electrodes formed on the n-type epitaxial layer and p-type contact electrodes formed on the p-type epitaxial layer, wherein the sub-wavelength grating layer includes gratings made of silicon material and having a specific pattern. The device provided by the invention has the advantages of easy integration, high quantum efficiency in a wide spectrum range, and high frequency response bandwidth; and meanwhile, relative technology is low in cost, simple and easy to implement. The problem that quantum efficiency and frequency response bandwidth of a traditional semiconductor optical detector are mutually restricted is solved, and the sub-wavelength grating reflection enhanced silicon-based wide-spectrum integrated optical detector and the preparation method thereof can be widely used in fields such as optical communication and optical signal processing.