The invention discloses a micro-longhole measuring method based on a fiber grating, relating to the field of size measurement into a micro inner cavity. The method mainly reflects the contact condition between a wall-touch spherule and the inner wall of a millipore to be measured through the drift value of the central wavelength of a fiber grating reflectance spectrum and realizes the three-dimensional aperture measurement of the millipore to be measured by the aid of a control circuit and a three-coordinate measuring engine. The micro-longhole measuring method based on the fiber grating, which is provided by the invention, has the advantages of low loss, good spectrum characteristics, high accuracy and good repetitiveness without null drift, expands the measurement range, converts contact conditions in space into the change of an optical spectrum, overcomes the limitation caused by diffraction loss in measuring the millipore to be measured by using a traditional CCD (Charge-Coupled Device) spot displacement method, avoids the influence of the conditions, such as the power fluctuation of a light source, the microbending effect of fiber, the coupling loss, and the like, avoids causing loss to the micro hole and prolongs the service life of an optical probe.