The invention discloses a mask plate and a preparation method of an optical waveguide reflecting surface and an optical waveguide, and relates to the technical field of the optical transmission structure and manufacturing. Control of the light transmittance of the lithography process is realized by the specially designed mask plate. When the ultraviolet light penetrates through the mask plate, thelithography depth of the positions of different light transmittance is different so as to form a slope structure. Meanwhile, diffraction of light is formed between two adjacent micro light transmission areas so that the slope obtained by lithography is a smooth reflecting surface rather than a concave-convex step surface. The method of forming the reflecting surface through lithography by using the mask plate is easy and feasible, and the mask plate also has the characteristics of simple structure, low cost and easy preparation so that the preparation process of the optical waveguide reflecting surface can be greatly simplified and the preparation cost of the reflecting surface can be reduced by the method, and the method has extremely high generalization and is an important advance in comparison with the methods in the prior art.