A double-longitudinal-mode NdYVO4 laser with ultra-large frequency difference is reported, which can be used for generating narrowband terahertz (THz) wave radiation. Continuous-wave (CW) double-longitudinal-mode operation of the laser is achieved based on the thin-disk laser medium and the micro-cavity, and the frequency difference between the two longitudinal modes is in the THz band, tuned by adjusting the length of the laser cavity and varying within the range of 0.1~0.2 THz; The spectral linewidth of each longitudinal mode is as low as 20 MHz; The spatial distribution of the laser beam is a single transverse mode with the beam quality factor M2 of 1.292; The output power of the CW laser is 116 mW with the slope efficiency of 18.53% and the linear polarization degree of 0.993, when the power of the pumping light is 630 mW. Used as the seed light, the double-frequency output light of the double-longitudinal-mode laser will be amplified to obtain the high-power THz difference-frequency pumping light, which can realize the narrowband THz-wave radiation via collinear difference frequency generation in the nonlinear crystal, while the theoretical analysis indicates the spectral linewidth of the THz-wave expands to approximately 2 times than that of the THz difference-frequency pumping light, and using the THz difference-frequency pumping light with spot radius of 50 μm and peak power in kilowatt magnitude and 0.8 mm DAST crystal, the THz-wave radiation with output power in milliwatt magnitude can be generated.