In this paper, the semi-cladding waveguides with diameters of 30, 40 and 50 μm were successfully prepared in lithium niobate (LiNbO3) crystals by femtosecond laser micro-nano-processing technology. Their waveguiding performances were tested by the end-face coupling technique, which results in the waveguides having excellent transmission performances at the wavelengths of 633 and 1 550 nm. The guidance is valid only for TM polarization with the single-polarized nature. By calculating the transmission loss of the waveguide, it is concluded that the 30 μm diameter waveguide has the best transmission performance at 633 and 1 550 nm wavelengths. The experimental results are proved to be reasonable by theoretical calculations using Rsoft software. The results of this study provide an important reference for the design and performance optimization of optical waveguides, and have certain experimental and theoretical value.