The invention provides a single-frequency laser wavelength comparison device and a method. The device comprises a first single-frequency laser, a second single-frequency laser, an optical beam combiner, a nondegenerate optical cavity, a dispersion element, a photoelectric detector and an oscilloscope. By adopting the above device, the method comprises the following steps: output light of two lasers is combined on the optical beam combiner, and two light beams are enabled to have the same light beam parameters and the same propagation directions after the optical beam combiner; one light beam is blocked, and the other light beam is adjusted to realize mode matching with the nondegenerate optical cavity; and the wavelength of the other laser beam is changed, and the waveform different between the two light beams is compared through comparing the mode matching efficiency between the two light beams and the nondegenerate optical cavity. The device and the method are simple, convenient, practical, low in cost and good in practical values.