In order to measure micro-cavity dimensions precisely, an aiming and triggering measurement method by a double optical fiber coupler based on the location of Orthogonal Fourier-Mellin moments (OFMM’s) is proposed.By transmitting reversely the light between different optical fibers through the coupler，the method transforms the transverse displacement of the fiber probe into the displacement of laser, and the laser displacement then is transformed into a large displacement of a CCD by a lens.To improve accuracy, the rotation invariance and particular description capability of OFMM’s is used to locate the edge of image at subpixel, and a compensation based on OFMM’s real position is proposed to improve the precision of edge location.The location accuracy of OFMM’s and performance of sensor are experimentally measured.According to JJF(hei)8-2008, the proposed method with a researched micro-hole machine are used to measure the diameter of a blind micro-hole with the depth of 2000 μm and diameter of 200 μm, and the obtained repeatability uncertainty is less than 0.25 μm.