Lithium niobate thin film has become the preferred optical material in the field of tunable Fabry-Perot filter, electro-optic modulator and other devices towards integration and miniaturization because of its excellent optical properties and the advantages of easy integration with compact optical waveguides and other devices. However, due to the size difference between optical waveguide and optical fiber, there is a serious mode field mismatch, which leads to a large insertion loss when the optical fiber is coupled with optical waveguide. In this paper, as an ultra-high numerical aperture (UHNA) fiber is directly coupled with ridge waveguide, optical properties such as light field mode distribution, coupling efficiency and loss are analyzed by simulation. The results show that as the etching depth and width are 300 nm and 0.8 μm, respectively, the coupling efficiency between the LiNbO3 ridge waveguide and the single-mode fiber (UHNA7) can reach 33.8% and the coupling loss is 4.71 dB. The simulation also compares that when the upper cladding material is replaced with silicon dioxide and silicon nitride materials, the coupling efficiency value of the ridge waveguide and single-mode fiber is significantly increased to 63.4% and the coupling loss is reduced to 1.98 dB.