A precision measuring system for large size measurement is described. The frequency-stabilized He-Ne laser with thermoregulation is used as the light source, which operates on the double longitudinal modes, has high frequency stability and good anti-disturbance performance. The frequency stability is better than 10-7 (in open air). It has the beat frequency about 790 MHz and beat wavelength 376 μm. The node of the beat-wave is used as sampling flag and the distance between the node and the close measuring point is measured with double frequency interferometer, which is combined in the same optical system. In this system, an adaptive filter and a wavelet transform program are used to improve the measurement accuracy. The test indicates that the measuring accuracy is better than 30 μm/10 m and the measuring range is 0～20 m.