Abstract: A precision measuring method and device for large size work-piece is presented in the paper.The frequency-stabilized double longitudinal modes He-Ne laser with thermoregulation is used as light source,which has high stability of frequency difference(less than 1×10-7 in the open air) and good anti-disturbance performance that has been proved by theory and experiments.Its beat frequency is about 750MHz and beat wavelength 400mm.The node is used as sampling flag and the beat-wave fringe fraction is measured with double frequency interferometer combined by optical components.The experiments indicate that the accuracy of determining nodes is less than 0.05mm,the resolution for measuring fringe fraction is 0.08μm,the accuracy of the device is ±50μm+1.5×10-6·L(L is the measured distance,unit m)in the range of 20m length.