The invention provides a method for measuring the depth of a loose ring of a deeply-buried columnar jointed rock tunnel. The method comprises the steps that firstly, holes are formed in the rear portion of a tunnel face of the tunnel and used for installing Bragg fiber grating sensors; the interface of the loose ring of the deeply-buried columnar jointed rock tunnel is measured according to readings of the Bragg fiber grating sensors, and then the depth of the loose ring is obtained; fracture pressure and reopening pressure of cylindrical surfaces on the two sides of the interface of the loose ring are measured through a cyclic pressure water test method; by analyzing the difference value of the measured fracture pressure and the measured reopening pressure, the depth of the loose ring of the columnar jointed rock tunnel is further determined accurately. The method is combined with a testing method to conduct correction mutually, data measured by the grating sensors are monitored and analyzed by means of the optical principle, and measured data are accurate, intuitive and easy to distinguish; furthermore, the loose interface and depth are determined accurately according to data, measured by the grating sensors, of the loose interface and depth of the columnar jointed rock tunnel through pressure water testing.