A simple method is presented to determine small wavelength differences based on the dispersion properties of a uniaxial crystal and circularly polarized heterodyne interferometry. A circularly polarized heterodyne light beam is incident on a uniaxial crystal at the Brewster's angle, and the reflected light beam passes through an analyzer for interference. Owing to proper azimuth angles of the transmission axis of the analyzer and the optical axis of the crystal, the variation of the phase difference determined with the heterodyne interferometric technique of the interference signal is significantly enhanced, resulting in an accurate wavelength variation. The feasibility of this method was demonstrated, and the sensitivity of wavelength differences is about 0.001 nm. The proposed approach has a simple structure, straightforward operation, high stability, and high sensitivity.