Difference-frequency generation enables optical nonlinear-wavelength conversion and is vital for generating coherent light at long wavelengths. In this study, we demonstrate modal-phase-matched difference-frequency generation on a thin-film lithium-niobate platform. By varying the wavelength of the signal light, difference-frequency generation is realized across the entire spectrum of the signal laser. In addition, by manipulating the power of the signal light coupled into the waveguide, an on-chip conversion efficiency of 188 %·W^-1 is achieved. Experimental results show that the proposed method is promising for multiwavelength frequency conversion in optical communication and future all-optical communication networks.