Abstract: A largesize device precissily measuring spares and being able to be applied on site t is studied in the paper．The frequencystabilized HeNe laser with thermoregulation is used as the light source, which operates on the double longitudinal modes and 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 frequency wavelength λ=380μm which is benchmark of measurement. 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, the large size L is measured with the resolution of 0.08μm. The test indicates that the device has a high environment adaptability and highest frequency difference stability among the same kind of existing great size device.The measuring accuracy is better than ±30μm/10m and the measuring range is 0～20m. This results is important to resolve the ambivalent promble between environment and measure precision of large size in petroleum machine.

Key words: large size measurement, double longitudinal mode laser, beat frequency difference, high precision, petroleum machine