In accordance with the complicated app lication environment of blade and the high measurement precision requirements of blade tip clearance,a detecting technique and the relevant system,which use dual frequency laser with large frequency difference, have been designed and tested.On the basis of phase laser rangfinder principle, we take use of dual frequency laser with large frequency difference to gain high frequency and modulated light,and then we ado pt optical fiber transmission structure into the system to avoid some environment distractions.On top of these,we hav e desi gned the high-frequency weak optical signal processing system.Finally,we use all phase FFT (pFFT) to obtain the phase difference betwee n the measuring light and the reference light,and inverse the tip clearance according to the derived formula.The model analysis shows that the measuring accuracy is mainly dependent on the accuracy of phase difference and measuring ruler rather than the environment al factors,for example the shape of blade,the number of blades or electromagnetic interference.Thus,the system can realize static an d dynamic calibration and measurement,as well as solve the dynamic calibration problems and external interferences in non-contac t high speed rotating blade tip clearance measurement under the harsh environments.In the clearance measurements,the accuracy is up t o 34.26μm,and in repeat experiments,the standard deviation of phase difference is 0.2572°.