An absolute distance heterodyne interferometer using a dual-longitudinal mode He-Ne laser is proposed. In this interferometer an optical heterodyne technique is used to enhance the signal/noise ratio of its opto-electrical signals. By measuring the phase difference between these two heterodyne signals, the fractional fringe of the synthetic wavelength is measured precisely and directly. Also the influence on the measuring accuracy from the mode coupling is analyzed. The compensation for this error is discussed. An accuracy of 140 μm in 25 m is obtained.